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THE 


SYDENHAM SOCIETY 


ENSTLREURED 


MDCCCXLIII 


LONDON 


MDCCCXLVII. 


MICROSCOPICAL RESEARCHES 


INTO THE 


ACCORDANCE IN THE STRUCTURE AND GROWTH 


or 


ANIMALS AND PLANTS. 


TRANSLATED FROM THE GERMAN 
OF 


DR. TH. SCHWANN 


PROFESSOR IN THE UNIVERSITY OF LOUVAIN, 
ETC. ETC. 


BY 


HENRY SMITH 
FELLOW OF THE ROYAL COLLEGE OF SURGEONS OF ENGLAND, 
SURGEON TO THE ROYAL GENERAL DISPENSARY, ALDERSGATE STREET, 


LONDON 
PRINTED FOR THE SYDENHAM SOCIETY 


MDCCCXLVITI. 


TRANSLATOR’S PREFACE. 


Any attempt on my part by way of introduction or com- 
mendation of Professor Schwann’s work, must, I feel, be 
altogether misplaced and unnecessary. The treatise has now 
been seven years before the public, has been most acutely in- 
vestigated by those best competent to test its value, and the 
first physiologists of our day have judged the discoveries 
which it unfolds as worthy to be ranked amongst the most 
important steps by which the science of physiology has ever 
been advanced. The Roya Socizty or Lonpon has evinced 
its sense of the great merit of the work by awarding to its 
Author the Coprey Mepat for the year 1845. The exten- 
sive reputation and fully-acknowledged value of the original 
work, then, forbid my presuming that any one of my readers 
can be altogether unacquainted with it and the general 
outlines of the Crrti-THrory; I may, however, I trust, be 
permitted to add a few words respecting the edition which is 


now presented to the Subscribers of the Sydenham Society. 


In the first place, I desire to tender my most unfeigned 
and unreserved apologies to the Council and Subscribers of 
the Society for the delay which has occurred in the issuing 


of this translation, and to assure the latter body that their 
b 


vi TRANSLATOR’S PREFACE. 


Council is m no degree responsible for its tardy appearance ; 
when, nearly three years since, the Council did me the honour 
to accept an offer on my part to present to the Society a 
translation of Professor Schwann’s treatise, I fully hoped to 
have proceeded with so pleasing a labour without interruption 
or hinderance; but various unforeseen circumstances, both of a 
professional and domestic nature, have occurred to prevent the 


accomplishment of my object until the present moment. 


I am greatly indebted to the Author for the labour which 
he has expended in revising his work for this translation. 
Amongst the most important advantages which this edition 
has derived from his revision, I may mention the addition of 
many notes illustrative of the text, and the amalgamation of 
the two papers on Cartilage and Ossification, which, as they 
were originally written and printed at a considerable interval 
of time, led to some difficulty in the comprehension of the 
Author’s precise views on that subject; and that cireum- 
stance is also to be received as explanatory of the appearance 
of some of the delineations of Cartilage in Plate III. It was 
originally intended to have added notes, which should bring 
down the history of the subject to the period of publication, 
but it was found that they would form a mass of material 
almost as large as the original text, and the idea was therefore 


abandoned. 


In order that the reader might be in possession of the whole 
of the evidence upon which the Cell-Theory was originally 
based, I have appended a translation of Dr. ScurzipEn’s 


Monograph so frequently referred to by our Author. 


TRANSLATOR’S PREFACE. Vil 


It is to be feared that many of my readers may consider 
an apology to be necessary on my part for the style of the 
translation, and think that I might have followed the German 
less closely with advantage; the nature of the subject, how- 
ever, involving as it does such very minute descriptions, and 
the constant repetition of the same terms, added to the im- 
possibility of doing justice to the Author’s close deductions in 
any other form than a literal translation, necessitated a much 
more rigid adherence to the original text, than I should have 


thought requisite under any other circumstances. 


The Plates have been most faithfully copied from the 
originals by Mr. Henry Adlard. 


HENRY SMITH. 


HENRIETTA STREET, CAVENDISH SQUARE; 
November 30th, 1847. 


AUTHOR’S PREFACE. 


Ir is one of the essential advantages of the present age, 
that the bond of union connecting the different branches of 
natural science is daily becoming more intimate, and it is to 
the contributions which they reciprocally afford each other that 
we are indebted for a great portion of the progress which the 
physical sciences have lately made. This circumstance there- 
fore renders it so much the more remarkable, that, notwith- 
standing the many efforts of distinguished men, the anatomy 
and physiology of animals and plants should remain almost 
isolated, though advancing side by side, and that the conclu- 
sions deducible from the one department should admit only of - 
a remote and extremely cautious application to the other. Of 
late, the two sciences have for the first time begun to be more 
and more intimately allied. The object of the present treatise 
is to prove the most intimate connexion of the two kingdoms 
of organic nature, from the similarity in the laws of develop- 
ment of the elementary parts of animals and plants. 

The principal result of this investigation is, that one com- 
mon principle of development forms the basis for every sepa- 
rate elementary particle of all organised bodies, just as all 
crystals, notwithstanding the diversity of their figures, are 
formed according to similar laws. I have endeavoured to 
explain the design of such a comparison more fully in the 
commencement of the third section of this treatise, and will 
now lay before the reader those data which are of most im- 
portance in an historical point of view im reference to the 
development of this idea. 


x AUTHOR’S PREFACE. 


As soon as the microscope was applied to the investigation 
of the structure of plants, the great simplicity of their struc- 
ture, as compared with that of animals, necessarily attracted 
attention. Whilst plants appeared to be composed entirely 
of cells, the elementary particles of animals exhibited the 
greatest variety, and for the most part presented nothing at 
all in common with cells. This, harmonised with the opinion 
long since current, that the growth of animals, whose tissues 
are furnished with vessels, differed essentially from that of 
vegetables. An independent vitality was ascribed to the 
elementary particles of vegetables growing without vessels, 
they were regarded to a certain extent as individuals, which 
composed the entire plant; whilst, on the other hand, no such 
a view was taken of the elementary parts of animals. An 
essential difference both in the mode and in the fundamental 
powers of growth was thus maintained. 

It soon, however, appeared that animal tissues do also 
occur which grow without vessels; for instance, in the forma- 
tion of the ovum, and the earlier stages of development of the 
embryo previous to the formation of the blood; and, secondly, 
certain tissues of the adult, the epidermis for example. With 
respect to the ovum, which manifested indubitable proofs of 
an actual vitality, all physiologists were agreed in ascribing to 
it a so-called plant-like growth. This resemblance to the plant 
had reference to a growth of the conspicuous parts of the ovum 
without vessels, and was in no way connected with the form 
and mode of growth of the elementary particles. No one, 
however, considered that the analogy of the ovum entitled him 
to infer the operation of a plant-like growth of the elementary 
particles in the non-vascular tissues of the matured animal ; 
on the contrary, the opinion rather gained ground, that these 
tissues originated and grew by means of a secretion from the 
surface of the organised tissues. Such was supposed to be 
the case with the epithelium, the crystalline lens, &c. This 


AUTHOR’S PREFACE. XI 


opinion still maintained its ground, even when the structure 
of the tissues became more accurately known. Nor did the 
plant-like growth of the component parts of the ovum abolish 
the assumed essential difference of the growth of the vascular 
tissues. 

A very important advance was made in the year 1887, 
when an actual growth of the elementary particles of epithe- 
lium was proved to take place without vessels. Henle (Sym- 
bole ad anatomiam vill. intest. Berol. 1837) showed that the 
cells in the superficial layers of epithelium are much more ex- 
panded than those in the deeper strata, a fact which leaves 
scarcely any doubt as to their true plant-like (i. e. non-vascular) 
growth. Henle’ says (l. c. p. 9), “Hoe in loco (in planta 
pedis) cellularum (retis Malpighii) diametrum extrorsum 
augeri, sepius repetita observatione pro re certa affirmare 
audeo. Quas retis cellulas non minus in fceetu suillo sensim 
increscentes transire in cellulas epidermidis, nunquam non 
inveni.” Purkinje and Raschkow (Meletem. circa mammal. 
dentium evol. Vratisl. 1835) had made the following obser- 
vations upon the development of the epidermis: “In primis 
evolutionis periodis—squamule—epithelii nondum ita con- 
formate sunt ut in illa periodo, que partui precedit, sed 
parenchyma plantarum cellulis simillimum ostendunt, cum 
quzque squamula, que postea talis apparet, tunc temporis 
tanquam cellula polyedrica e membrana tenacissima constans 
globosamque guttulam continens in conspectum veniat. Pressu 
applicato rumpebantur istz cellule atque lymphaticum liquo- 
rem effundebant, que cellulz, procedente evolutione, verisimile 
complanatz in illas polyedricas squamas mutantur.” Henle, 
when quoting this passage, adds (l. c. p. 9): “ Hee illa num 
vero sola compressio in causa esse possit, ut parva cellula 


' Henle’s observations are detailed at page 76 of this treatise. The researches of 
Turpin and Dumortier could not be quoted, as I only became acquainted with them 
at the conclusion of my work. 


xii AUTHOR’S PREFACE. 


in tantam laminam extendatur, nondum satis mihi constat : 
certe principio increscere volumen cellulze, nescio an imbibitione, 
constabit, nisi spes fallit, promotis disquisitionibus.” The 
caution with which Henle (and, indeed, every good physiolo- 
gist) expresses himself in this passage with reference to the 
true growth of non-vascular tissues, is the best illustration of 
the state of the question. There is another observation of 
Henle’s, which is opposed to the epithelium being regarded as 
a lifeless substance secreted from the organised tissue; I allude 
to the passage (1. c. p. 22 et seq.) where he proves that the 
vibratile cilia, whose motion it is so difficult to explain by 
physical laws, stand upon little cylinders which are merely a 
modification of the epithelium. 

Turpin (Annal. des Sciences natur. vu, p. 207) showed that 
the corpuscles, which Donné had found in vaginal discharges, 
and regarded as cast-off epithelium, were organised cells, and 
were in general oblong, and either pointed at one or both 
ends, or altogether irregular in figure, and that a new gene- 
ration of spherical vesicles’ took place in their interior. He 
then remarks (1. c. p. 210): ‘On ne peut s’empécher, aprés 
avoir bien étudié les vésicules dont est formée la couche de 
mucus produite par la membrane muqueuse vaginale, d’y voir 
un tissu cellulaire bien organisé et composé comme tous les 
tissus cellulaires végétaux, d’un agglomérat, par simple conti- 
guité, de vésicules distinctes et vivant individuellement chacune 
pour leur propre compte au dépens de eau muqueuse, qui les 
baigne de toutes parts.” Turpin then compares this tissue of 
animal cells, presented under the appearance of mucus, with 
what he calls “ suppurations végétales, excrétions muqueuses, 
qui semblent suinter sous forme de gouttelettes, de la surface 
des tissus vifs,’ and which is generally comprised under the ~ 


' May there not have been some confusion here with the nuclei of the epithelium- 
cells? At present, as far as regards Mammalia at least, we know of no formation 
of cells within cells in the epithelium. 


AUTHOR’S PREFACE. Xili 


name of cambium ; and finally adds (1. c. p. 212), “ En étendant 
ja comparaison entre deux choses si comparables, on trouve 
que la forme variable des vésicules du tissu cellulaire du mucus 
de la membrane vaginale, leur allongement en pointe, leur 
flaceidité, toujours entretenue par Vhumidité constante qui 
baigne les tissus animaux, et le développement dans leurintérieur, 
soit des granules, soit des vésicules sphériques, sont toutes 
choses qui s’observent également dans la composition de tous 
les tissus cellulaires végétaux mous et aqueux, et que l’on 
désigne par le nom de pulpe ou de parenchyme dans certaines 
tiges ou feuilles grasses et dans certains fruits murs ou blettes.” 
In the same year, Dumortier communicated researches into 
the development of the ova of snails. (Annal. des Sciences 
natur. vill, p. 129.) He observed, that in the mucus-globule, 
present in these ova, and from which the embryo is developed, 
there are generated cells, in the interior of which, secondary 
cells are formed, and so on, and that this tissue of cells be- 
comes transformed into the liver, whilst the other tissues origi- 
nate from a gelatinous mass, which exhibits myriads of points. 
In his conclusions, he says (l. c. p. 163), “En examinant 
Pévolution des Mollusques, nous avons démontré que les tissus 
animaux, quoique formés originairement de méme par la solidi- 
fication des surfaces, se développent de différentes manieéres : 
le tissu cellulaire par des productions médianes, le tissu dermo- 
musculaire par un feutré de canalicules centripétes. Ainsi, chez 
les animaux, les tissus ne se forment pas au dépens les uns des 
autres ; il n’y existe pas un tissu générateur unique, mais bic 
plusieurs tissus originairement distincts.—Les belles observa- 
tions de M. Mirbel ont prouvé que chez les végétaux il existe 
un seul tissu originel, le tissu cellulaire, qui par une suite de 
métamorphoses, se transforme en tissu vasculaire. Par con- 
séquent, le régne végétal est caractérisé par Punité originel, et 
le regne animal par la pluralité originelle des tissus.” Van- 


beneden and Windischmann give a different explanation to 


XiV¥ AUTHOR'S PREFACE. 


these observations of Dumortier, in as much as they regard the 
tissue consisting of cells as the yelk and not the liver. (Bulletin 
de l’Acad. royale de Bruxelles, tom. v, No. 5.) 

Other instances of the resemblance in form between different 
animal tissues and those of vegetables had already been 
repeatedly pointed out. Thus it was frequently said, in refer- 
ence to thickly-crowded animal cells, or even mere globules, 
that they presented an appearance resembling vegetable cellu- 
lar-tissue ; and Valentin (Nov. Act. N. C. xvui, P. 1, 96), 
after describing the nucleus of the epidermal cells, states that 
it reminded him of the nucleus which occurs in the vegetable 
kingdom, in the cells of the epidermis, the pistil, &. Nothing, 
however, resulted from such comparisons, because they were 
mere similarities in figure, between structures which present 
the greatest variety of form. 

Schleiden instituted researches into the mode of development 
of vegetable cells, which illustrated the process most excellently. 
This admirable work appeared subsequently in the second part 
of Miiller’s Archiv for 1838. He found, that in the forma- 
tion of vegetable cells, small, sharply-defined granules are first 
generated in a granulous substance, and around them the cell- 
nuclei (cytoblasts) are formed, which appear like granulous 
coagulations around the granules. The cytoblasts grow for a 
certain time, and then a minute transparent vesicle rises upon 
them, the young cell, so that, in the first instance, it is placed 
upon the cytoblast, like a watch-glass upon a watch. It then 
becomes expanded by growth. Schleiden communicated the 
results of his investigations to me, previous to their publication 
in October, 1837. The resemblance in form, which the chorda 
dorsalis, to which J. Miiller had already drawn attention, and 
the branchial cartilage of the tadpole present to vegetable cells, 
had previously struck me, but nothing resulted from it. The 
discoveries of Schleiden, however, led to more extended re- 
searches in another direction. 


AUTHOR’S PREFACE. XV 


In the above-mentioned investigations of Henle, Turpin, and 
Dumortier, the resemblance which the animal tissues examined 
(epithelium and the liver or yelk of snails) bore to plants, lay, 
in the first place, in the circumstance, that their elementary 
particles grew without vessels, and in part, free in a fluid, or 
even inclosed in another cell; and in the second place, in that 
these elementary particles exhibiting a non-vascular growth, 
were furnished with a peculiar wall, like the cells of plants. 
When this coincidence was furnished, we were entitled to 
arrange these cells as near to the vegetable cells as the different 
kinds of animal cells, for instance, germinal vesicles, blood-cor- 
puscles, and fat-cells, stand together, when regarded as different 
species comprised under the natural-history idea of cells. 

The state of the matter, therefore, when I commenced my 
researches was as follows: The elementary particles of or- 
ganised bodies presented the greatest variety of form; there 
was a resemblance between many of them, and, according to 
the greater or lesser degree of similarity, a group of fibres, of 
cells, of globules, and so on, might be distinguished, and 
in each of these divisions again there were different forms. 
As the cells taken collectively differed from the fibres, so also, 
only in a less degree, must the separate kinds of cells differ from 
each other, and the different kinds of fibres from each other. 
All those forms seemed to have nothing else in common, save 
that they grew by the addition of new molecules between those 
already existing, that they were living elements. So long as 
the epithelium-cells were regarded as a secretion of the 
organised substance, they could never, in that sense, be classed 
with the living elementary particles. There seemed to be no 
general rule with respect to the mode in which the molecules 
were joined together to form the living particles; here they 
united into one kind of cells, there into another, and at a third 
spot into a fibre, and so on. The principle of development ap- 
peared to be altogether different for such particles as differed 


XVi AUTHOR’S PREFACE. 


in their physiological signification; and the diversity m the 
laws which it was necessary to assume in the development of 
a cell and a fibre, was also, only in a less degree, necessarily 
assumed between the different kinds of cells and the different 
sorts of fibres. Cells, fibres, &e. were therefore merely natural- 
history ideas, and no conclusion could be drawn from the mode 
of development of one kind of cell as to that of any other kind ; 
and, in fact, no such deductions were made, although we were ac- 
quainted with some important points in the process of develop- 
ment of certain kinds of cells ; for example, the blood-corpuscle 
(see p. 67 of this Treatise), and the ovum (see the Supplement, 
p. 217). Although the investigations quoted above determined 
the important fact of the non-vascular growth, they did not 
thereby effect any change in our views. The idea of proving 
the similarity of the principle of development for elementary 
particles which were physiologically different, by a comparison 
of animal cells with those of vegetables, was not contained in 
those researches, and with these, therefore, the mvestigators 
before mentioned might well come to a stand-still. 

The discoveries of Schleiden made us more accurately ac- 
quainted with the process of development in the cells of plants. 
This process contained sufficient characteristic data to render 
a comparison of the animal cells in reference to a similar 
principle of development practicable. In this sense I com- 
pared the cells of cartilage and of the chorda dorsalis with 
vegetable cells, and found the most complete accordance. The 
discovery, upon which my inquiry was based, immediately lay 
in the perception of the principle contained in the proposition, 
that two elementary particles, physiologically different, may 
be developed in the same manner. For it follows, from the 
foregoing, that if we maintain the accordance of two kinds of 
cells in this sense, we are compelled to assume the same princi- 
ple of development for all elementary particles, however dis- 
similar they may be, because the distinction between the other 


AUTHOR’S PREFACE. XVil 


particles and a cell differs only in degree from that which exists 
between two cells; so also the principle of development in the 
latter can only then be similar, when it repeats itself in the 
rest of the elementary particles. I therefore quickly asserted 
this position also, so soon as I was convinced of the accordance 
between the cells of cartilage and those of plants in this sense. 

It now became easy to accommodate the principle which I 
had laid down to the rest of the tissues, since the principle 
itself had already made me acquainted with the law of their 
development. Actual observation also completely confirmed 
the conclusion which had been drawn with respect to the rest 
of the tissues. It was not absolutely necessary that this 
principle should recur in the elementary particles of vascular 
tissues; for since no independent vitality of the elements, 
and therefore no diversity in the fundamental powers of 
growth, was assumed in their case, so, without prejudice to 
the principle, might they be subject to entirely different laws 
of development. But slight as was the probability at the 
commencement, that the principle could be carried out with 
respect to them, observation soon showed that vessels do not 
establish any essential difference in growth, but merely occa- 
sion some distinctions, which may be explained as the con- 
sequences of a more minute distribution of the nutrient fluid ; 
of the change of material facilitated both by that means and 
by the circulation; and of a greater capacity of imbibition 
in the animal substance. Thus was the proposition firmly 
established by observation, that there is one common principle 
of development for the elementary particles of all organised 
bodies. It had already indeed been long known that all 
tissues were formed from a granulous mass; but that these 
granules bore some direct relation to the subsequent ele- 
mentary particles, and what that relation might be was known 
in respect to but a few of the particles, and in them the mode 
of development appeared to differ so much, that unity neither 


xVill AUTHOR’S PREFACE. 


was nor could be recognised in it; for the conformity of the 
principle of development consists chiefly in the similar origin of 
these granules themselves, and this circumstance was not known, 
indeed the term granules or granulous mass was sometimes 
used to denote the entire cells, sometimes the nuclei, and some- 
times granulous substances which form to a certain extent 
as chemical precipitates, and have no direct connexion with 
the elementary cells of organised bodies. 

I communicated a preliminary review of the results gained, 
and which already comprehended most of the tissues, in the 
beginning of the year 1838, in Froriep’s ‘ Notizen,’ Nos. 91, 
103, and 112. The detailed description required a longer 
time ; the first two portions of the present Treatise were placed 
before the Academy of Paris in August and December, 1838. 
J. Miller and Henle have already applied the theory to the 
most important pathological processes, and it now only requires 
to be extended to comparative anatomy, particularly amongst 
the lower animals. 

At the conclusion of the Treatise I have attempted a theory 
of organisms, and for that purpose have excluded everything 
theoretical from the work itself, in order that facts might not 
be confused with hypothetical matter. The theory has at 
least this advantage, that by its aid any one may form a pre- 
cise idea for himself of the organic processes, which may con- 
duct to new researches; such a theory may therefore be of 
use, even if assumed to be decidedly false. It contains the 
principles of the organic phenomena, both of the healthy and 
diseased organism. It was my intention to have added an 
application of the theory to the several organic processes ;_ but 
circumstances compelled me to bring the work to a conclusion. 
Perhaps at some future time I may find opportunity to fill up 
the deficiency. 


Berlin, March 1839. 


CONTENTS. 


PAGE 
INTRODUCTION ; 3 3 ‘ 4 ; 1 


SECTION I. 
On the Structure and Growth of the Chorda Dorsalis and Cartilage : = le 
1. Chorda dorsalis. : : : ° 5 wn 
2. Cartilage . : , : > - yt) 


SECTION II. 


On Cells as the Basis of all Tissues of the Animal Body . c « 36 
First division. On the ovum and germinal membrane . : - 40 
Second division. Permanent tissues of the animal body : . 64 

Cuass I. Isolated, independent cells Z z ‘ ‘ = 167 

1. Lymph-corpuscles : - : : 5 lee 
2. Blood-corpuscles 5 = “ 5 toy 
3. Mucus-corpuscles : 5 - 5 5 
4. Pus-corpuscles . < : ey it 

Cuass II. Independent cells united into continionk tissues ‘ - éd 

1. Epithelium é é : . . Sy 4 tite 
2. The pigmentum nigrum : : : ae 
3. Nails . : : 3 : : a 80 
4. Hoofs : - - : : >) ell 
5. Feathers : ; : : 4 4 thy 
6. The crystalline lens. : 87 


Cuass III. Tissues, in which the cell-walls arn coalesced with Sich einen 


or with the intercellular substance. : 3 5 ale 
1. Cartilage and bone. : ; ; ie i 
2. The teeth - ib. 

Crass IV. Fibre-cells, or tissues, wean tietints from valle that Tecdane 

elongated into bundles of fibres. : - - 110 
1. Areolar tissue. : : ; : Atle 
2. Fibrous tissue 5 : - ; a alee 
3. Elastic tissue . : - é . 124 


XX CONTENTS. 


PAGE 
Crass V. Tissues generated from cells, the walls and cayities of which coalesce 
together : ; : : : . 129 
1. Muscle : : : : : ; 30 
2. Nerves : : : ; ea 
3. Capillary yeeele : 5 4 j . 154 
SECTION III. 
Review of the previous researches—The formative process of Cells—The 
Cell-theory 7 E : é : . 26 
Survey of Cell-life : : 5 : : . 168 
Theory of the cells . : : : : : +, 86 


Supplement (vide page 46) on the signification of the germinal membrane . 217 
Remarks upon a statement put forth by Professor Valentin, respecting pre- 


vious researches on the subject of this work A = ene 
Explanation of the Plates : ? s : 5 . 225 
CONTRIBUTIONS TO PHYTOGENESIS, EI Dr. M. J. ScHLEIDEN - 229 


Description of Plates to do. : : 5 + 0260 


MICROSCOPICAL RESEARCHES, 


&e. &e. 


INTRODUCTION. 


AxtuovueH plants present so great a variety of external form, 
yet they are no less remarkable for the simplicity of their 
internal structure. This extraordinary diversity in figure is 
produced solely by different modes of junction of simple ele- 
mentary structures, which, though they present various modi- 
fications, are yet throughout essentially the same, namely, cells. 
The entire class of the Cellular plants consists only of cells ; 
many of them are formed solely of homogeneous cells strung 
together, some of even a single cell. In like manner, the Vas- 
cular plants, in their earliest condition, consist merely of simple 
cells; and the pollen-granule, which, according to Schleiden’s 
discovery, is the basis of the new plant, is in its essential parts 
only a cell. In perfectly-developed vascular plants the struc- 
ture is more complex, so that not long since, their elementary 
tissues were distinguished as cellular and fibrous tissue, and 
vessels or spiral-tubes. Researches on the structure, and par- 
ticularly on the development of these tissues, have, however, 
shown that these fibres and spiral-tubes are but elongated cells, 
and the spiral-fibres only spiral-shaped depositions upon the 
internal surface of the cells. Thus the vascular plants consist 
likewise of cells, some of which only have advanced to a higher 
degree of development. The lactiferous vessels are the only 
structure not as yet reduced to cells; but further observations 
are required with respect to their development. According to 
Unger (Aphorismen zur Anatomie und Physiol. der Pflanzen, 
Y 1 


2 INTRODUCTION. 


Wien, 1838, p. 14,) they in like manner consist of cells, the 
partition-walls of which become obliterated. 

Animals, which present a much greater variety of external 
form than is found in the vegetable kingdom, exhibit also, and 
especially the higher classes in the perfectly-developed condition, 
a much more complex structure in their individual tissues. 
How broad is the distinction between a muscle and a nerve, 
between the latter and cellular tissue, (which agrees only in 
name with that of plants,) or elastic or horny tissue, and so 
on. When, however, we turn to the history of the development 
of these tissues, it appears, that all their manifold forms originate 
likewise only from cells, indeed from cells which are entirely 
analogous to those of vegetables, and which exhibit the most 
remarkable accordance with them in some of the vital pheno- 
mena which they manifest. The design of the present treatise 
is to prove this by a series of observations, 

It is, however, necessary to give some account of the vital 
phenomena of vegetable cells. Each cell is, within certain 
limits, an Individual, an independent Whole. ‘The vital phe- 
nomena of one are repeated, entirely or in part, in all the rest. 
These Individuals, however, are not ranged side by side as a 
mere Aggregate, but so operate together, in a manner unknown 
to us, as to produce an harmonious Whole. The processes 
which go forward in the vegetable cells, may be reduced to the 
following heads: 1, the production of new cells; 2, the expan- 
sion of existing cells; 8, the transformation of the cell-contents, 
and the thickening of the cell-wall; 4, the secretion and ab- 
sorption carried on by cells. 

The excellent researches of Schleiden, which throw so much 
light upon this subject, form the principal basis for my more 
minute observations on these separate vital phenomena. (See his 
“ Beitrage zur Phytogenesis,” in Miuller’s Archiv, 1838, p. 137, 
plates 3 and 4.)1 

First, of the production of new cells. According to Schleiden, 
in Phenogamous plants, this process always (except as regards 
the cells of the Cambium,) takes place within the already ma- 
ture cells, and in a most remarkable manner from out of the 
well-known cell-nucleus. On account of the importance of the 


1 [A translation of this paper forms part of this volume.—Trawns. ] 


INTRODUCTION. 3 


latter in reference to animal organization, I here introduce an 
abridgment of Schleiden’s description of it. A delineation is 
given in plate I, fig. 1, a, a, taken from the onion. This struc- 
ture—named by R. Brown, Areola or cell-nucleus, by Schleiden, 
Cytoblast—varies in its outline between oval and circular, ac- 
cording as the solid which it forms passes from the lenticular 
into the perfectly spheroidal figure. Its colour is mostly yel- 
lowish, sometimes, however, passing into an almost silvery 
white; and in consequence of its transparency, often scarcely 
distinguishable. It is coloured by iodine, according to its 
various modifications, from a pale yellow to the darkest brown. 
Its size varies considerably, according to its age, and according 
to the plants, and the different parts of a plant in which it is 
found, from 0:0001 to 0:0022 Paris inch. Its internal struc- 
ture is granular, without, however, the granules, of which it 
consists, being very clearly distinct from each other. Its 
consistence is very variable, from such a degree of softness as 
that it almost dissolves in water, to a firmness which bears 
a considerable pressure of the compressorium without altera- 
tion of form. In addition to these peculiarities of the cyto- 
blast, already made known by Brown and Meyen, Schleiden has 
discovered in its interior a small corpuscle (see plate I, fig. 1, 4,) 
which, in the fully-developed cytoblast, looks like a thick ring, 
or a thick-walled hollow globule. It appears, however, to pre- 
sent a different appearance in different cytoblasts. Sometimes 
only the external sharply-defined circle of this rmg can be dis- 
tinguished, with a dark point in the centre,—occasionally, and 
indeed most frequently, only a sharply circumscribed spot. In 
other instances this spot is very small, and sometimes cannot 
be recognized at all. As it will frequently be necessary to speak 
of this body in the following treatise, I will for brevity’s sake 
name it the “nucleolus,” (Kernkorperchen, “nucleus-corpuscle.”) 
According to Schleiden, sometimes two, more rarely three, or, 
as he has personally informed me, even four such nucleoli occur 
in the cytoblast. Their size is very various, ranging from the 
semi-diameter of the cytoblast to the most minute point. 

The following is Schleiden’s description of the origin of the 
cells from the cytoblast. So soon as the cytoblasts have attained 
their full size, a delicate transparent vesicle, the young cell, 
rises upon their surface, and is placed upon the fiat cytoblast 


4 INTRODUCTION. 


like a watch-glass upon a watch. It is at this time so delicate 
that it dissolves in distilled water in a few minutes. It gradu- 
ally expands, becomes more consistent, and at length so large, 
that the cytoblast appears only as a small body inclosed in one 
of the side walls. The portion of the cell-wall which covers the 
cytoblast on the inner side, is, however, extremely delicate and 
gelatinous, and only in rare instances to be observed; it soon 
undergoes absorption together with the cytoblast, which like- 
wise becomes absorbed in the fully-developed cell. The cyto- 
blasts are formed free within a cell, in a mass of mucus-granules, 
and the young cells lie also free in the parent cell, and assume, 
as they become flattened against each other, the polyhedral 
form. Subsequently the parent cell becomes absorbed. (See a 
delineation of young cells within parent cells, plate I, fig. 2, 
6, 6,6.) It cannot at present be stated with certainty that the 
formation of new cells always takes place from a cystoblast, and 
always within the existing cells, for the Cryptogamia have not 
as yet been examined in this respect, nor has Schleiden yet ex- 
pressed his views in reference to the Cambium. Moreover, 
according to Mirbel, a formation of new cells on the outside of 
the previous ones takes place in the intercellular canals and on 
the surface of the plant in the Phanerogamia. (See Mirbel on 
“ Marchantia,” in Annales du Musée, 1, 55; and the counter- 
observations of Schleiden, Miiller’s Archiv, 1838, p. 161.) A 
mode of formation of new cells, different from the above de- 
seribed, is exhibited in the multiplication of cells by division of 
the existing ones ; in this case partition-walls grow across the 
old cell, if, as Schleiden supposes, this be not an illusion, inas- 
much as the young cells might escape observation in conse- 
quence of their transparency, and at a later stage, their line 
of contact would be regarded as the partition wall of the parent 
cell. 

The expansion of the cell when formed, is, either regular on 
all sides, in which case it remains globular, or it becomes poly- 
hedral from flattening against the neighbouring cells, or it is irre- 
gular from the cell growing more vigorously in one or in several 
directions. What was formerly called the fibrous tissue, which 
contains remarkably elongated cells, is formed in this manner. 
These fibres also become branched, when different points of 
the cell-wall expand in different directions. This expansion of 


INTRODUCTION. D 


the cell-wall cannot be explained as a merely mechanical effect, 
which would continually tend to render the cell-membrane 
thinner. It is often even combined with a thickening of the 
cell-wall, and is probably effected by that process of nutrition 
called intus-susception. (See Hugo Mohl’s “ Erlauterung und 
Vertheidigung memer Ansicht von der Structur der Pflanzen- 
substanzen,” Tiibingen, 1836.) - The flattening of the cells may 
also be ascribed to the same cause. 

With regard to the changes which the cell-contents and cell- 
wall undergo during vegetation, I only take into consideration 
the thickening of the latter, as I have but a few isolated obser- 
vations upon the transformations of the contents of animal cells, 
which however indicate analogous changes to those of plants. 
The thickening of the cell-walls takes place, either by the depo- 
sition from the original wall, of substances differmg from, or 
more rarely, homogeneous with it, upon the internal surface of 
the cell, or by an actual thickening of the substance of the cell- 
wall. The first-mentioned form of deposition occurs in strata, 
at least this may be distinctly seen in many situations. (See 
Meyen’s Pflanzen-Physiologie. Bd. 1, tab. I, fig. 4.) Very 
frequently,—according to Valentin, universally,—these deposi- 
tions take place in spiral lines ; this is very distinct, for example, 
in the spiral canals and spiral cells. The thickening of the cell- 
membrane itself, although more rare, appears still in some in- 
stances indubitable, for imstance, in the pollen-tubes, (e. g. 
Phormium tenax.) Probably that extremely remarkable phe- 
nomenon of the motion of the fluid, which has now been ob- 
served in a great many cells of plants, is connected with the 
transformation of the cell-contents. In the Charz, in which it 
is most distinct, a spiral motion may also be recognized in it. 
But, for the most part, the currents intersect each other in the 
most complex manner. 

Absorption and Secretion may be classed as external ope- 
rations of the vegetable cells. The disappearance of the parent 
cells in which young ones have formed, or of the cell-nucleus 
and of other structures, affords sufficient examples of absorption. 
Secretion is exhibited in the exudation of resin in the intercel- 
lular canals, and of a fluid containing sugar by the nectar- 
glands, &c. &e. 

In all these processes each cell remains distinct, and main- 


6 INTRODUCTION. 


tains an independent existence. Examples, however, also occur 
in plants, where the cells coalesce, and this not merely with 
regard to their walls, but the cavities also. Schleiden has found 
that in the Cacti, the thickened walls of several cells unite to 
form a homogeneous substance, in which only the remains of 
the cell-cavities can be distinguished. PI. I, fig. 3, represents 
such a blending of the cell-walls observed by Schleiden. The 
entire figure is a parent cell, with thickened walls, in which 
four young cells have formed, the walls of which are likewise 
thickened and have coalesced with each other, as well as with 
those of the parent cell; so that only the four cavities remain 
with their nuclei in a homogeneous substance. The spiral ves- 
sels, and, according to Unger, the lactiferous vessels also, afford 
examples of the union of the cavities of several cells by the 
absorption of the partition walls. 


After these preliminary remarks we pass on to animals. The 
similarity between some individual animal and vegetable tissues 
has already been frequently pointed out. Justly enough, how- 
ever, nothing has been inferred from such individual points of 
resemblance. Every cell is not an analogous structure to a ve- 
getable cell; and as to the polyhedral form, seeing that it neces- 
sarily belongs to all cells when closely compacted, it obviously 
is no mark of similarity further than in the circumstance of 
densely crowded arrangement. An analogy between the cells of 
animal tissues and the same elementary structure in vegetables 
can only be drawn with certainty in one of the following ways : 
either, Ist, by showing that a great portion of the animal tissues 
originates from, or consists of cells, each of which must have 
its particular wall, in which case it becomes probable that these 
cells correspond to the cellular elementary structure univer- 
sally present in plants; or, 2dly, by proving, with regard to 
any one animal tissue consisting of cells, that, m addition 
to its cellular structure, similar forces to those of vegetable 
cells are in operation in its component cells ; or, since this is im- 
possible directly, that the phenomena by which the activity of 
these powers or forces manifests itself, namely, nutrition and 
growth, proceed in the same or a similar manner in them as in 
the cells of plants. I reflected upon the matter in this point of 
view in the previous summer, when, in the course of my re- 


INTRODUCTION. 7 


searches upon the terminations of the nerves in the tail of the 
Larve of frogs (Medic. Zeitung, 1837), I not only saw the beau- 
tiful cellular structure of the Chorda Dorsalis in these larve, but 
also discovered the nuclei in the cells. J. Miiller had already 
proved that the chorda dorsalis in fishes consists of separate cells, 
provided with distinct walls, and closely packed together like 
the pigment of the Choroid. The nuclei, which in their form 
are so similar to the usual flat nuclei of the vegetable cells that 
they might be mistaken for them, thus furnished an additional 
point of resemblance. As however the importance of these 
nuclei was not known, and since most of the cells of mature 
plants exhibit no nuclei, the fact led to no farther result. 
J. Miiller had proved, with regard to the cartilage-corpuscles 
discovered by Purkinje and Deutsch in several kinds of cartilage, 
from their gradual transition into larger cells, that they were 
hollow, thus in a more extended sense of the word, cells; and 
Miescher also distinguishes an especial class of spongy cartilages 
of a cellular structure. Nuclei were likewise known in the 
cartilage-corpuscles. Miiller, and subsequently Meckauer, 
having observed the projection of the cartilage-corpuscles at 
the edge of a preparation, it became very probable that at least 
some of them must be considered as cells in the restricted sense 
of the word, or as cavities inclosed by a membrane. Gurlt also, 
when describing one form of permanent cartilage, calls them 
vesicles. I next succeeded in actually observing the proper wall 
of the cartilage-corpuscles, first in the branchial cartilages of 
the frog’s larvee, and subsequently also in the fish, and also the 
accordance of all cartilage-corpuscles, and by this means in 
proving a cellular structure, in the restricted sense of the word, 
in all cartilages. During the growth of some of the cartilage- 
cells, a thickening of the cell-walls might also be perceived. 
Thus was the similarity in the process of vegetation of animal 
and vegetable cells still further developed. Dr. Schleiden oppor- 
tunely communicated to me at this time his excellent researches 
upon the origin of new cells in plants, from the nuclei within 
the parent-cell. The previously enigmatical contents of the cells 
in the branchial cartilages of the frog’s larve thus became 
clear to me; I now recognized in them young cells, provided 
with a nucleus. Meckauer and Arnold had already found fat- 
vesicles in the cartilage-corpuscles. As I soon afterwards suc- 


8 INTRODUCTION. 


ceeded in rendering the origin of young cells from nuclei 
within the parent-cells in the branchial cartilages very pro- 
bable, the matter was decided. Cells presented themselves in 
the anima body having a nucleus, which in its position with 
regard to the cell, its form and modifications, accorded with 
the cytoblast of vegetable cells, a thickening of the cell-wall 
took place, and the formation of young cells within the parent- 
cell from a similar cytoblast, and the growth of these without 
vascular connexion was proved. ‘This accordance was still 
farther shown by many details; and thus, so far as con- 
cerned these individual tissues, the desired evidence, that these 
cells correspond to the elementary cells of vegetables was fur- 
nished. I soon conjectured that the cellular formation might 
be a widely extended, perhaps a universal principle for the 
formation of organic substances. Many cells, some having 
nuclei, were already known; for example, in the ovum, epi- 
thelium, blood-corpuscles, pigment, &c. &c. It was an easy 
step in the argument to comprise these recognized cells under 
one point of view; to compare the blood-corpuscles, for example, 
with the cells of epithelium, and to consider these, as likewise 
the cells of cartilages and vegetables, as corresponding with each 
other, and as realizations of that common principle. ‘This was 
the more probable, as many points of agreement in the progress 
of development of these cells were already known. C. H. Schultz 
had already proved the preexistence of the nuclei of the blood- 
corpuscles, the formation of the vesicle around the same, and 
the gradual expansion of this vesicle. Henle had observed the 
gradual increase in size of the epidermal cells from the under 
layers of the epidermis, towards the upper ones. The growth 
of the germinal vesicle, observed by Purkinje, served also at first 
as an example of the growth of one cell within another, although 
it afterwards became more probable that it had not the signi- 
fication of a cell, but of a cell-nucleus, and thus furnished proof 
that everything having the cellular form does not necessarily 
correspond to the cells of plants. A precise term for these 
cells, which correspond to those of plants, should be adopted ; 
either elementary cells, or vegetative cells (vegetations-zellen). 
By still further examination, I constantly found this principle 
of cellular formation more fully realized. The germinal mem- 
brane was soon discovered to be composed entirely of cells, and 


INTRODUCTION. 9 


shortly afterwards cell-nuclei, and subsequently also cells were 
found in all tissues of the animal body at their origin ; so that 
all tissues consist of cells, or are formed by various modes, from 
cells. The other proof of the analogy between animal and vege- 
table cells was thus afforded. 

I shall follow the same course in communicating the separate 
observations, and shall speak, therefore, in the next place of the 
structure and growth of the chorda dorsalis and cartilage, and 
in the second section treat of the germinal membrane and the 
remaining tissues. 


10 STRUCTURE AND GROWTH 


SECTION I. 


ON THE STRUCTURE AND GROWTH OF THE CHORDA DORSALIS 
AND CARTILAGE. 


1. Chorda Dorsalis. 

Tux Chorda Dorsalis in the larve of frogs and fishes les 
in, or in some instances, under the bodies of the vertebrae, and 
is continued behind the coccyx, through the whole length of 
the tail. It is imclosed by a firm sheath, and forms a spindle- 
like, consistent, gelatiniform, transparent cord, which is thick- 
est at the commencement of the tail, and thence gradually 
diminishes in each direction, both towards the skull and the 
point of the tail. It cannot well be separated entire in re- 
cently killed animals, but is best obtamed from them in the 
form of delicate transverse sections. If the animal be placed 
in water for twenty-four hours or longer after death, and the 
tail then severed from the body at their point of junction, the 
chorda dorsalis may be entirely pressed out, by gently scraping 
along its course from the point of the tail, or from the head, 
towards the wound. As this does not succeed if the animal be 
allowed to lie out of water for the same period after death, the 
easier separableness appears to depend upon a penetration of the 
water between the chorda dorsalis and its sheath ; the firmer con- 
nexion of it in the fresh condition, however, only upon a more 
close contact, or wedging in of the chorda dorsalis, and not upon 
a vascular connexion, for I do not suppose that it contains any 
vessels. | Microscopically examined, it exhibits, as J. Miller 
has discovered in fishes, a cellular structure in its interior, sur- 
rounded externally by a proportionately thin cortical substance 
(rinde), which is beset with scattered granules. The interior 
exactly resembles the parenchymatous cellular tissue of plants. 
(See plate I, fig. 4.) It is readily seen, especially at the point 
of contact of three cells, that each one is surrounded by its 
own proper membrane. The cells vary much in size, being 


OF THE CHORDA DORSALIS. 1] 


usually largest in the centre, and becoming somewhat smaller 
towards the outside. They have an irregular polyhedral shape, 
mostly with spherical surfaces, which are sometimes convex 
towards the outside, sometimes towards the cavity of the cell. 
Their walls are very thin, colourless, smooth, and almost 
completely transparent, firm, and slightly extensible. They 
dissolve readily in caustic potash. The rudiments of the 
chorda dorsalis in the conical interstices of the vertebra of 
cartilaginous fishes are not dissolved by dilute or concentrated 
acetic acid. The chorda dorsalis of fishes according to J. Miller 
does not become converted into gelatine after long boiling. 
The cells of the chorda dorsalis of frog’s larve contain in their 
interior a colourless, homogeneous, transparent fluid, which 
does not become cloudy at a boiling heat; the slight clouding 
observed in the chorda dorsalis after boiling, appears to be 
situated more in the cell-walls, which afterwards appear mi- 
nutely granulated. 

In the larva of Pelobates fuscus another formation occurs, 
inasmuch as by far the greater proportion of these cells contain 
a very distinct nucleus. It has the appearance of a somewhat 
yellowish-coloured small disc, of a roundish oval form, rather 
smaller than a blood-corpuscle of the frog, and almost as flat. 
(See plate I, fig. 4a, where it is represented from the chorda dor- 
salis of Cyprinus erythrophthalmus.) In frog’s larve the nucleus 
is nearly twice as large. It has a sharp, dark margin, and ap- 
pears minutely granulated. In this little disc may be seen one, 
rarely two, and very seldom three dark, sharply circumscribed 
spots. It thus entirely resembles, both as a whole as well as in 
its modifications, the cytoblast of vegetable cells with its nu- 
cleolus, and microscopically, cannot at all be distinguished from 
it. Compare plate I, fig. 4a, with plate I, fig. la. But it also 
corresponds with it in its position in the cell. In very many 
cells, the vertical wall of which is viewed from above, it may 
be seen that the nucleus lies close on the inner surface of the 
wall of the cell, or even embedded in the wall. It appears 
then, as in plate I, fig. 1 a’, only still somewhat flatter. I 
have not, however, succeeded in observing that a lamella of 
the cell-wall passes over its internal surface, which is also but 
rarely seen in plants. If the external minutely granulated 
cortical substance of the chorda dorsalis of Pelobates fuscus 


12 STRUCTURE AND GROWTH 


be more accurately examined, it is found that the granules are 
oval, and furnished with a nucleolus, and that, with the excep- 
tion of their being only about half as large, they entirely re- 
semble the cell-nuclei. This cortical substance is not sharply 
separated from the proper tissue of the chorda dorsalis; and as 
the cells of the latter suddenly diminish very much towards the 
cortical substance, I think that these granules upon the latter 
are the cytoblasts of flattened cells which form it. Sometimes, 
although but indistinctly even with a very favorable light, very 
fine lines may be perceived in the intermediate spaces between 
these granules, where the cells come in contact, as in the 
common tabular (or scaly) epithelium. In the chorda dorsalis 
of the larva of Rana esculenta, where the nuclei in the cells 
are not distinct, these nuclei in the cortical substance are not 
seen ; the tabular structure, however, is evident in them. One 
must be very cautious in denying the presence of the cyto- 
blasts, when they are not immediately recognizable. They 
may in animals, as in plants, atta such a degree of trans- 
parency, as renders them very difficult of observation. Thus, 
I could not for a long time detect them in the rudiment of the 
chorda dorsalis, which is found in the conical mtermediate 
spaces of the vertebra, in a large Carp, until on a very clear 
day they appeared very pale but quite recognizable, and of pre- 
cisely the form above described. They were somewhat more 
distinct in the Pike and Cyprinus erythrophthalmus. The 
delineation, plate I, fig. 4, is taken from the latter. They 
are however smaller in these fishes than in frog’s larve. 

To return to the larva of Pelobates fuscus. Here the cells 
of the chorda dorsalis lie so close to each other, that the walls 
of the two neighbouring cells are in immediate contact. Even 
when three or more cells are in contact, they are generally so 
close, that only the contiguous walls are observable. Some- 
times, however, in such instances, a small intermediate space 
remains, which is larger than could be filled up by the unthick- 
ened cell-wall; and there is then seen, as in plants, a species 
(apparent or real?) of intercellular substance, or an intercel- 
lular canal. With regard to this latter (intercellular canal), occa- 
sionally, though rarely, in such an instance of close contiguity 
of three cells, upon making a transverse section, the cell-walls are 
observed sharply bounded, as well towards the cell as externally, 


OF THE CHORDA DORSALIS. 13 


and between the cells a small triangular interstice is seen, which 
is filled by a transparent fluid (not by air), or at least by a sub- 
stance which refracts the light in a different manner from the 
cell-walls, just as it is represented in plate I, fig. 1c, from the 
onion. 

Young cells, which float free, form within the cells of the 
chorda dorsalis, as in plants. They are, however, in the larve 
of the frog so transparent, that very favorable light and good 
instruments are required to see them. The number of cells, 
also, in which new ones are formed in the larvee is not great, at 
least in such as are to be had in the latter part of autumn. In 
the above-mentioned species of Cyprinus, and also in other 
fishes, they are, however, easy to be seen, and in greater number. 
Vesicles of very various sizes may be perceived in the cavities 
of many of these cells, and also in those of the larvee of the 
frog, though they are more difficult of observation in the latter ; 
a single one of these vesicles sometimes fills the greater part 
of the cavity; and occasionally several lie in one cell. (PI. I, 
fig. 4, 6, 6, c.) They are commonly quite round; but not 
unfrequently two are in contact, and flattened against each 
other. That they le free in the cell, follows from the fact, 
that they may be isolated without rupture. If, for instance, 
a small portion of the chorda dorsalis be torn into minute 
pieces, and a thin plate of glass with water be placed upon 
them, by moving this lightly backwards and forwards a few 
times, some such isolated vesicles may often be brought into 
the field of vision. They may then be made to roll about, and 
thus demonstrate their globular form. I have taken great 
pains to discover a nucleus in their walls, but without suc- 
cess. The young cells of the chorda dorsalis, also, in the larvee 
so often mentioned, have often the appearance, so long as 
they are not isolated, of possessing a nucleus: but one may 
readily be deceived here, since such a nucleus may belong to 
a cell lymg above or below them. Caution must also be 
used, not to confound a globular epithelial cell, which may have 
shpped into the chorda dorsalis in making the transverse sec- 
tion, with the true cells of that structure. I have not as yet 
been able, with certainty, to observe any nucleus, at least not 
of the characteristic form, in isolated young cells of the chorda 
dorsalis. In rare instances, a very small corpuscle, (d, d, of 


14 STRUCTURE AND GROWTH 


the figure,) lay in the inner surface of the young cell. It must 
remain a question whether the nucleus is really wanting, or 
whether it is only not visible in consequence of its translucency, 
or whether these corpuscles are developed into the nucleus. 
The chorda dorsalis accords with the vegetable cells, at least in 
this respect, that young cells are formed within the old ones. 

With regard to the thickening of the cell walls; these ap- 
pear to remain always simple (unchanged) in the chorda dorsalis 
of the larva of the frog. But im the fully developed osseous 
fishes, in Cyprinus, for example, a thickening is exhibited in 
those cells which le near the axis of the conical interspaces 
of the vertebre. The cell-cavities always become smaller in 
consequence of this thickening of the walls. The thickened 
walls, or the intermediate substance between the cell cavities 
consist of closely cohering longitudinal fibres, between which 
very fine transverse fibres are also sometimes seen. The longi- 
tudinal fibres run uninterruptedly past several cells; and the 
primitive membrane of each cell can no longer be distinguished. 

To sum up the researches upon the chorda dorsalis in a few 
words ; it may be said to consist of polyhedral cells, which have, 
in or on the internal surface of their walls, a structure, according 
in its form and position with the nucleus of the cells of plants, 
namely, an oval flat disc containing one, two, or more rarely 
three nucleoli. The cells usually le in close contact with each 
other; but sometimes at points where three or more cells meet 
together, a sort of intercellular substance, or an intercellular 
passage is seen. Young cells, which are at first round, and float 
free, are formed within parent cells. Nuclei of the charac- 
teristic form, are not distinctly observed in these young cells, 
but sometimes a small globule lies upon their inner surface. In 
those cells which undergo further development, the cell-mem- 
brane ceases to exist as a distinct structure, and the interme- 
diate substance between the cell cavities consists for the most 
part of longitudinal fibres. 

With the exception of the formation of these fibres, into the 
origin of which I have not yet examined, and the absence of the 
nucleus in the young cells, these cells entirely accord with the 
vegetable cells. It must remain undecided whether the nu- 
cleus is really wanting in these young cells, as it is not yet 
proved to exist in all plants, (for example in many acotyledo- 


OF CARTILAGE. 15 


nous plants,) or whether the little corpuscle, which presents 
itself on the inner surface of some young cells, is the nucleus 
which grows with the cell, as it is observed to do in some other 
animal cells; or whether the nucleus in the young cells is in- 
visible in consequence of its translucency, since even fully-deve- 
loped cells are met with, in which, although certainly present, 
it is, in consequence of its transparency, barely visible. 


2. Cartilage. 


The accordance of the structure of cartilage with the tissue of 
plants is of more importance in reference to animal organization. 
We have here to do not only with a more widely extended animal 
tissue, but also with one which, at least, in its subsequent stages 
of development, contains vessels, and therefore bears more 
decidedly the character of an animal tissue. The simplest form of 
cartilage is exhibited in the cartilages of the branchial rays of 
fishes. If, for example, a branchial ray of Cyprinus erythroph- 
thalmus be loosened from the branchial arch, and the mucous 
membrane be removed by gentle scraping, the cartilage remain- 
ing presents the appearance of a little rod, which diminishes 
from the point of its imsertion on the branchial arch towards its 
free end, its sides being somewhat compressed, and exhibiting 
on their margins some blunt prominences. The structure of 
this cartilage is very simple. At the point it perfectly resembles, 
in its whole appearance, the parenchymatous cellular tissue of 
plants. (See pl. I, fig. 5, from the above-mentioned Cyp. 
eryth.) Little polyhedral cell-cavities with rounded corners are 
seen lying closely together. The cell-cavities are separated 
from each other by extremely thin partition walls. The cell- 
contents are transparent, and a small pale round nucleus (a) may 
be seen in some cells when in the recent state, in others only 
after the action of water upon them. The structure of the 
lateral prominences of the cartilage is similar to that at the 
point, only that the cells are somewhat extended in length. 
Advancing from that point towards the middle, or still better from 
the point towards the root of the branchial ray, the partition 
walls of the cell-cavities are observed to become gradually 
thicker ; and the cavities are here somewhat smaller. (PI. I, 
fig. 6.) On the thickened cell-walls it may now also be seen 


16 STRUCTURE AND GROWTH 


that the intermediate substance of the cell-cavities is not a 
simple structure, but one composed of the walls peculiar to the 
contiguous cells: that is to say, each cell is surrounded with a 
thick ring, its peculiar wall, the external outline of which is 
more or less distinct. In the preparation from which the deli- 
neation is taken, it was in some parts quite as distinct as the 
internal. Between two cells these external outlines blend into 
one line, but separate again when the contact of the cell-walls 
ceases ; there is thus often left between the cell-walls a three 
or four-cornered intermediate space (c), filled with a kind of 
intercellular substance. No other structure, no deposition of 
strata, or distinction between primary cell-membrane and se- 
condary deposit can be observed in the thickened cell-walls. 
The cell-contents also remain clear after the thickening of the 
walls. At the base of the branchial ray, it is scarcely possible 
to distinguish between the different cells-walls, and the cartilage 
presents the appearance of a homogeneous substance, in which 
separate small cavities only are seen. (Pl. I, fig. 7.) Around 
some few only of the cell-cavities, a trace of the peculiar cell-— 
walls may be seen in the form of a ring. ‘This ring is usually 
somewhat thin, so that the entire intermediate substance of the 
cell-cayities cannot be formed of the cell-walls ; but the inter- 
cellular substance, which was so small in quantity in the centre 
of the branchial ray, here contributes essentially to the forma- 
tion of the cartilaginous substance, and often completely pre- 
vents the immediate contact of the cell-walls. This intercel- 
lular substance appears, however, to be homogeneous with that 
of the cell-walls, and in most situations coalesces with them. 
The cell-cavities, which are here transparent and without gra- 
nulous contents, are now the cartilage-corpuscles. 

The process of formation of this cartilage is as follows. It 
consists originally of cells, which lie in very close contact, but 
every one of which has its special, very thin cell-membrane. 
This follows, firstly, from the complete accordance in appear- 
ance, of cartilage in its earliest stage, with vegetable cellular 
tissue; secondly, from the presence of the nucleus in the 
young cells of cartilage, a structure which, as will subsequently 
be seen, occurs in almost all the cells proved to exist in other 
tissues ; thirdly, from the fact, that a separation of the cell- 
walls is often distinctly perceptible in instances where they are 


OF CARTILAGE. 7s 


thickened. These cell-walls lie either in close contact, or have 
only a trace of intercellular substance between them, or there 
is sufficient of that material to entirely prevent the contact of 
the different cells. Their walls, which are originally formed 
of a very thin membrane, become thickened. The cavities of 
the cells with thickened walls which are seen in the centre of 
the branchial ray, are smaller than those of the cells which le 
nearer the surface, the walls of which are less dense ;_ but, 
whether this is produced by a thickening of the cell-wall taking 
place from without inwards, or whether rather the cells were not 
smaller in their original formation, is a matter of uncertainty. 
No deposition of strata, nor any distinction from the primordial 
cell-membrane, can be recognized in these thickenings of the 
walls. The condensed cell-walls at length coalesce either with 
each other, or with the intercellular substance, to form one 
homogeneous mass, in which only the cell-cavities remain per- 
ceptible, presenting the appearance of small distinct excavations 
filled with a transparent substance; these cell-cavities are the 
cartilage-corpuscles. 

In the foregoing description no error can arise from the 
great variety in form which the cartilage-corpuscles frequently 
present ; for, on examining the branchial rays of a very large 
pike, the gradual transition may be traced, from the thin- 
walled almost globular cells to the most varied forms, in which 
the remains of the cell-cavities are so much extended in length 
as to give to the cartilage almost a fibrous appearance. 

The same extremely simple process of formation (modified, 
however, in some important respects) is presented in all carti- 
lages. These modifications, the fundamental type of which is 
already pointed out in the cartilages of the branchial rays of 
fishes above described, depend chiefly upon the share relatively 
contributed by the thickened cell-walls, or the intercellular sub- 
stance, to form the intermediate substance of the cell-cavities, 
or cartilage-corpuscles. We have seen that this intermediate 
substance was formed almost entirely of the thickened cell- 
walls, with but a minimum amount of intercellular substance, 
in the centre of the branchial rays of fishes, whilst at their base, 
that is, in the earliest formed cartilage, the intercellular sub- 
stance preponderated, and the less dense cell-walls contributed 
less to the formation of the true substance of the cartilage. 

2 


18 STRUCTURE AND GROWTII 


The walls of the cells appear to contribute little or nothing to 
the formation of the substance of most of the ossifying carti- 
lages,—those of the higher animals for example. 

The cartilages of the branchial arches of the tadpole, like 
those of the branchial rays of fishes, consist of cells, which are, 
however, much larger than those of the fish, though smaller 
than the cells of the chorda dorsalis, with which they have, in 
every other respect, much similarity. The partition-walls of 
the cells are thicker than in the chorda dorsalis, but they may 
still be termed thin when compared with the cell-cavities. (See 
pl. ILI, fig. 1, which exhibits branchial cartilage from the young 
larva of Pelobates fuscus.) The cartilage intended to be used for 
investigation must be taken quite fresh from the living animal ; 
for the structures become very indistinct if it be allowed to le 
in water for any time after death, even though it be entire. 
After stripping off the mucous membrane, the cellular structure 
is readily recognized by the aid of the microscope. The cells 
vary much in size, and are more or less flattened against one 
another. The wall of each separate cell may be distinctly seen 
in the majority of instances, and its thickness might even be 
measured ; that we cannot trace it so evidently in the smallest 
cells is probably referrible to the extreme thinness of their 
sides. The walls of the cells are for the most part in contact, 
but intercellular substance may be seen in many situations, 
and especially where several cells are contiguous. The surface 
of the cartilage, which is represented on the left and lower 
margin of the figure, (pl. III, fig. 1,) is formed in the first place 
of intercellular substance, which, in as much as the cells ori- 
ginate in it, may be called Cytoblastema. 

This cartilage may, therefore, be described as consisting of 
intercellular substance, or cytoblastema, in which great num- 
bers of cells are seen. The cell-contents are generally clear as 
water ; but in the younger and smaller ones (for example, c,) the 
contained matter is less pellucid, and somewhat granulous. Hach 
cell contains a spherical granulous nucleus, which lies upon the 
mner surface of the wall, and which again encloses a nucleolus. 
The size of the nucleus is not precisely alike in al! the cells : it is 
somewhat larger in the large ones, but its size bears no proportion 
to the increased bulk of the cell; and again, the smaller cells 
are not much larger than the nucleus which they contain. 


OF CARTILAGE, 19 


Nuclei, around which no cells have yet commenced to be de- 
veloped, may be observed in the cytoblastema between the 
cells in some situations ; for example, a and 6. These like- 
wise contain a nucleolus, and are somewhat less than the nuclei 
in the smaller cells. 

The above observations furnish us with a complete repre- 
sentation of the development of cartilage-cells, and show the 
accordance of that process with the development of vegetable- 
cells, inasmuch as they exhibit the simultaneous presence in the 
cytoblastema both of simple nuclei, and of cells containing a 
nucleus of similar shape and size upon the inner surface of 
their walls, and which may be observed in all stages of tran- 
sition, from such as are scarcely larger than the nucleus they 
contain, to such as are many times its size. Simple nuclei are 
first present, developed in the cytoblastema. When these have 
arrived at a certain size, the cell is formed around and closely 
encompassing them. The cell gradually expands, whilst the 
nucleus remains lying on a point of the inner surface of its 
wall. The nucleus, also, increases somewhat in size, but not in 
proportion to the expansion of the cell. Now these three hy- 
potheses may be assumed from the above facts; either the cell 
is first developed, and the nucleus subsequently, or both are 
developed simultaneously, or the nucleus is first developed, 
and then the cell around it. The first supposition, that the 
cells are developed earlier than the nuclei, is not possible, since 
in that case cells would be found at a certain period of deve- 
lopment without nuclei. The simultaneous development of a 
cell, together with its nucleus, as two distinguishable struc- 
tures, is equally impossible, for then we should observe a stage 
of development, at which as yet the cell and nucleus had not 
reached the size of the ordinary nucleus. In order to explain 
the above observations, we must, therefore, have recourse to 
the third supposition, viz. that the nucleus is first developed 
and then the cell around it. 

The form of the young cells depends upon the space allotted 
them for expansion. They are, therefore, either round or angular, 
according as the neighbouring cells permit of, or limit their re- 
gular expansion. ‘T'wo or more cells are often developed close to- 
gether in one intercellular space, and thus compress those already 
formed, and the intercellular substance on the outside of them ; 


20 STRUCTURE AND GROWTH 


this fact explains the common appearance of two or four cells 
lying together in a group, being separated from one another 
by thin walls, whilst between such groups and the neighbour- 
ing cells we see much more intercellular substance. 

The cells at first appear finely granulated, and not so trans- 
parent as in the more fully developed condition. The thick- 
ening of the cell-membrane takes place simultaneously with 
its expansion. One of the cells in pl. III, fig. 1, exhibits two 
nuclei, one of which, like those of all the other cells, has but 
one nucleolus, the other having two. It may be conjectured, 
that this second nucleus is destined to the formation of a young 
cell within the larger one. 

In the intercellular substance at e in the same figure (pl. III, 
fig. 1,) may be seen a small corpuscle, surrounded by a granu- 
lous and indistinctly circumscribed mass, the rest of the inter- 
cellular substance being smooth and homogeneous. ‘This is, 
perhaps, a nucleus in the act of formation, the nucleolus of 
which is already developed ; and when the granulous mass sur- 
rounding that structure has obtained a defined external boun- 
dary, it will form a nucleus. If such be the case, we have 
here an instance of accordance of the development of the germ 
itself with the formation of the nucleus of vegetable-cells ob- 
served by Schleiden. 

On examining the cartilage of the branchial arches of the 
tadpole in the more completely developed state, (pl. I, fig. 8,) 
we find the cells generally lymg in groups, so that two, three, 
or four lie close together, separated from other groups by 
thicker partition walls. The special walls of the individual 
cells are less distinct, but at several spots where three or more 
cells are in contact, for example, at a, the separation of the 
walls may yet be seen, and a trace of intercellular substance is 
also present ; the latter, however, is almost homogeneouswith the 
cell-walls. It may also be observed that the cell-walls are thicker 
in these situations than they are represented in pl. III, fig. 1. 
Some parallel lines may be seen at various spots in these con- 
densed cell-walls, and the thickening might, in such instances, 
be supposed to be really produced by a stratified deposition of 
the substance upon the internal surface of the cell-wall. But 
at the same time it must be remembered, that every partition- 
wall between two cells must consist of two layers, each of which | 


OF CARTILAGE. 2t 


corresponds to the wall of the corresponding cell. This appear- 
ance of strata, however, is observed only in the thick walls 
between two groups of cells, and as these groups probably ori- 
ginate by the formation of two or four cells within a parent 
cell, each half of the partition-wall between two groups must 
(presuming such to be the mode of their formation) consist 
again of two layers, the one of which corresponds to the wall 
of the parent cell, the other to that of the secondary cell, so 
that each partition-wall of two groups must consist of four 
layers. Although it does, indeed, appear that even a greater 
number of layers or strata are present, yet I must at the same 
time remark, that these observations are by no means sufii- 
ciently conclusive for the proof of a fact so important in refer- 
ence to the process of nutrition, and that I am so far from re- 
garding them as evidence of a stratified deposition of the sub- 
stance, as not to hold such a thing to be even probable. The 
appearance is probably an optical deception. As before stated, 
no distinction was found between primary cell-membrane and 
secondary thickening in the cartilages of the branchial rays of 
fishes, but it seemed that the cell-membrane had actually be- 
come thickened ; neither is there any such distinction to be 
observed in the branchial cartilages of the tadpole. 

If the above described groups be assumed to have had their 
origin by the formation of secondary cells within a primary 
parent one, in that case, secondary cells which completely fill 
the parent one have not been developed in all the primary cells, 
for isolated cells occur in the branchial cartilages of Pelobates 
fuscus, which are somewhat larger than the secondary ones, 
but smaller than the other primary cells, and remarkable also, as 
will be seen immediately, from their contents. 

The cells of the branchial cartilages of the larva of Pelobates 
fuscus just mentioned, contain within them one or more nuclei. 
(Pl. I, fig. 8, d.) These nuclei, which may be easily isolated, 
are either slightly oval, or perfectly globular, more or less 
granulous and yellowish, and apparently hollow. They contain 
one or two very distinct, round, dark nucleoli, which lie in 
their interior either close upon the wall, or very near to it. 
The nuclei (a portion of them at least) appear to lie free in 
the cell-cavity, for they may readily be isolated. The above 
mentioned primary cells of the larva of Pelobates fuscus in 


22 STRUCTURE AND GROWTH 


which none of these secondary cells, completely filling the 
parent one have been developed, contain very commonly 
several such nuclei, and also one or more young cells. PI. I, 
fig. 8, ff, represents such young cells from the branchial carti- 
lages of the larva of Rana esculenta. They are round vesicles 
containing a nucleus identical in form and size with those 
which lie free, but which is situated upon the internal surface 
of the wall, and never in the centre of the cell. This nucleus 
is never wanting in the young cells. The cells, however, vary 
much in size, some being scarcely larger than the nucleus they 
contain, others twice or thrice as large: From one to three 
such young cells, in various stages-of development, are com- 
monly found within the primary one, where they sometimes 
become flattened from want of space. As the figure represents, 
most of the secondary cells contain these young ones, and but 
few of them onlysimple nuclei (such as have no cell around them), 
in some of the young cells, indeed, a second somewhat paler 
nucleus appears. These young cells lie free within the primary 
cell, and may be isolated in the same manner as was described 
with regard to those of the chorda dorsalis. They appear in the 
first instance to be perfectly transparent ; but gradually obtain 
a granulous yellowish aspect, and it is remarkable, that the 
earliest formation of this yellowish deposit takes place generally 
if not constantly, in the neighbourhood of the nucleus. 

It will thus be seen that these young cells, (fat cells ?) which 
are formed within the true cartilage-cells, furnish us with a 
series of observations as regards their development, similar to 
that observed in the formation of the cartilage-cells themselves: 
namely, simple nuclei, cells closely encompassing those nuclei, 
and all the stages of transition up to the largest cells; but 
never have we met with these young cells without nuclei. So 
that the same conclusions might be arrived at with respect to 
the mode of their development, as were before with regard to 
that of the cartilage-cells, namely, that the nuclei are first 
formed, and around them the cells, precisely as in plants. The 
nucleus in these young cells, however, does not appear to in- 
crease in growth after the cell has once formed around it. 
The accordance in form between these and the young cells of 
vegetables is shown by comparing Plate I, fig. 8, with fig. 2, 6. 

The nucleus of the true cartilage-cells like that of vegetable- 


OF CARTILAGE. 23 


cells is subsequently absorbed. After the cartilages of the 
branchial rays of fishes have been exposed to the action of water, 
it is only in the young cells that the nuclei are visible; they 
are much more rarely seen in those cells of which the walls are 
already very much thickened. In many cells of the branchial 
cartilages of the tadpole, a small nucleus with aragged outline 
may be observed, which is probably the cytoblast of the cell 
in the act of undergoing absorption. These cytoblasts (nuclei) 
of the true cartilage-cells always lie in the cell-cavity, even 
when its wall is thickened, and it is impossible to distinguish 
whether they lie free or are still connected with the cell-wall. 
A twofold explanation is here possible: either the cytoblast 
separates from the wall after the formation of the cell-membrane 
is perfected, and falls free into the cavity (as occurs in plants), 
and at such period a secondary deposition of substance upon 
the cell-wall first commences ; or the thickening of the wall is 
due to an actual increase of the original cell-membrane, and in 
that manner the nucleus is pushed inwards, and may remain 
in connexion with the wall. If a secondary deposition of 
substance took place before the nucleus was disengaged from 
the cell-membrane, that body must be enclosed in the wall, 
and would not lie in the cell-cavity. As both these expla- 
nations are possible, it will be seen that no conclusion can be 
drawn from the position of the nucleus, as to whether the 
thickening of the cell-wall be a secondary deposition, or an 
actual growth of the cell-membrane. Sometimes a carti- 
lage-cell presents more than one nucleus; when in such a 
ease the original nucleus of the cell is absorbed, all those 
observed are probably the germs of new cells, which have not 
as yet commenced their development. The same fact is fre- 
quently observed in plants. The nuclei in the branchial 
cartilages of the tadpole have for the most part the same size ; 
some, however, which are probably not as yet perfectly formed, 
are smaller than others. It also often occurs that a nucleus 
is seen expanded to three or four times the usual size ; such 
instances might be mistaken for young cells without nuclei, 
but they may be readily recognized by their general aspect. 
They are more transparent and delicate, and exhibit one or 
two nucleoli, which are easily detected ; when two are present 
they are widely separated from one another. According to 


24 STRUCTURE AND GROWTH 


Schleiden, a similar enlargement of the nucleus also occurs in 
plants, thus affording a remarkable accordance in what seems 
a very unimportant circumstance. It appears to be a kind of 
abortion; for I have never yet seen a cell formed around such 
a nucleus. 

The cranial cartilages of the tadpole (Plate I, fig. 9) are dis- 
tinguished from the branchial by the smaller size of the cell- 
cavities, and the increased strength of the firm intermediate 
substance. The walls of the separate cells cannot now be 
traced, they appear to have coalesced with the intercellular 
substance, which is present in greater quantity. The cells lie 
in groups of two or four together, and it is very probable, that 
in this cartilage, each group is formed of cells, which have 
been developed in a parent cell; for some may be seen, for 
example at c, which do not as yet quite fill the original cell. 
Such an instance, however, is rarely so very distinct as not to 
admit of a doubt. There is a very striking similarity between 
the group a, fig. 9, and fig. 3, which represents four young 
vegetable cells developed in a parent cell, and the thickened 
walls of which have coalesced with one another and with those 
of the parent cell, so that the four cavities only remain in an 
homogeneous substance. That portion of the cell-cavities 
which is still visible is filled with a granulous yellowish sub- 
stance, in which lie one or more nuclei, or young cells provided 
with a nucleus: these remains of the cell-cavities are the car- 
tilage-corpuscles discovered by Purkinje. 

The intercellular substance is universally much more pro- 
minent in the cartilages of mammalia than it is im those 
hitherto described, and in them it forms the principal part of 
the firm mass of the cartilage. There is not, however, any 
essential difference either between the structure of the several 
kinds of cartilage of mammalia, or between these and the car- 
tilage of lower animals, the only distinction being that it is a 
little more difficult to prove the existence of the special walls 
of the cartilage-cells in the former. 

The intercellular substance in some cartilages of mammalia 
is at first so soft, that the cells fall apart under slight pressure, 
and float free in the fluid. If, for example, a thin lamella be 
cut off from the cartilage at the angle of the lower jaw of a 
foetal pig of three and a half inches in length (a period when 


OF CARTILAGE. 25 


the cartilage is about to become, but is not as yet, ossified), and 
placed under the compressorium, the cells will be seen to lie so 
closely in it, that the space occupied by them may be estimated 
at three fourths, and that of the intercellular substance at one 
fourth of the whole volume. Many of the cells which have 
become separated by the process of cutting, float already in the 
fluid; and on shghtly compressing the preparation many more 
become loose, and flow out in streams from the intercellular 
substance into the surrounding fluid. The intercellular sub- 
stance is too soft to prevent the separation, but at a subsequent 
period of development this cannot be effected. According to 
Meckauer the cartilage-corpuscles may also be isolated by boil- 
ing. I once succeeded in crushing one of these young carti- 
lage-cells while still in connexion with the preparation. The 
first effect of the compressorium was to produce an extension 
of breadth; it then suddenly shrank together, whilst a clear 
fluid streamed out, thus proving the contents of the cell to be 
fluid and transparent. Now, inasmuch as these cells present in 
different instances a more or less granulous appearance, it fol- 
lows that the cells of ossifying cartilage must have a peculiar 
investing membrane, which is granulous, and thus that they 
are actual elementary cells, in our sense of the word, and nei- 
ther mere excavations in the substance, nor perfectly solid 
corpuscles. The appearance of the cells which float about en- 
tirely accords also with this view, for while their contents seem 
to be clear, the cells look granulated. All of them contain a very 
beautiful oval or circular, not flattened cell-nucleus, situate 
upon the internal surface of the wall, and this nucleus en- 
closes one or two very distinct nucleoli ; in short, they in every 
respect accord with the elementary cells of most of the other 
tissues. By the aid of acetic acid we may also frequently suc- 
ceed in rendering the cell-walls visible upon a thin lamella of 
cartilage, and as the cell-contents are at the same time dis- 
solved by the acid, it has the additional advantage of bringing 
the nucleus into view, which is sometimes indistinct in conse- 
quence of the granulous nature of the contents. Plate ITI, fig. 
2, exhibits a portion of cartilage so treated with acetic acid ; 
it is taken from the as yet unossified portion of the ilium of an 
embryo pig of five inches in length. The cell-walls, with their 
double outlines, may be seen, and both the illuminated and 


26 STRUCTURE AND GROWTII 


dark side in the thickness of the walls distinguished. The 
delineation, at the same time, proves how important a share is 
taken by the intercellular substance in the formation of the 
firm structure of cartilage. 

The cartilages of the foetus do not altogether accord in chemi- 
cal constitution with those of the adult, since we can obtain from 
them by boiling but a small quantity of a gelatinous substance, 
and that only with great difficulty, and they afford no true 
gelatine (capable of forming a jelly). I boiled some unossi- 
fied cartilages, consisting of apophyses of the femur and carti- 
laginous portions of the scapule, taken from several embryo 
pigs, measuring three and a half inches in length. After 
twelve hours’ boiling, they entirely crumbled into very small 
scales, which gave a variegated appearance to water im which 
they were stirred about, and appeared under the microscope 
extremely thin and granulous. The fluid, when filtered and 
evaporated almost to dryness, did not coagulate. Alcohol pro- 
duced a copious precipitate, which was dried, afterwards dis- 
solved in boiling water, and then evaporated almost to dryness; 
still no coagulation took place. Alum, however, clouded the 
fluid, and acetic acid had the same effect, but in a much 
slighter degree. As the quantity of cartilage made use of in 
the foregoing experiment was too small, I made a further in- 
vestigation with cartilage which had already become ossified, 
from the same embryos, namely, the frontal and parietal bones, 
scapule, humerus, femur, and some ribs. The unossified parts 
were removed as cleanly as possible from al] the bones. The 
earthy matter was withdrawn by hydrochloric acid; the carti- 
lages were then washed with water, and boiled for twenty-four 
hours. Under this process they fell to pieces very slowly , 
meanwhile numerous little glittermg scales appeared in the 
fluid, which, after beimg dried, resembled very minute fish- 
scales, and exhibited a beautiful play of colours. They were, 
perhaps, the lamellee described by Deutsch, which surround the 
minute medullary canaliculi. The form of most of the pieces 
of cartilage remained perfectly recognizable, and was but 
slightly altered. They looked of a yellowish-white colour, and 
not at all gelatinous, as substances usually do when about to 
be transformed into gelatine. The fluid was filtered from these 
little scales and pieces of cartilage, and then evaporated almost 


OF CARTILAGE. a7 


to dryness. It did not exhibit any trace of coagulation after 
standing twenty-four hours. After being dried, it was again 
dissolved in boiling water, on which occasion, however, a por- 
tion remained undissolved. It was, therefore, filtered; the 
fluid was copiously precipitated by alum, and the precipitate 
was, for the most part, although not entirely, dissolved, on the 
addition of alum in excess. Acetic acid likewise rendered the 
fluid very turbid, and an excess of acid did not entirely remove 
the cloudiness. It was copiously precipitated by tincture of 
gall-nuts, and acetic acid removed this precipitate again, leaving 
a very slight turbidness. (Acetic acid likewise completely dis- 
solves the precipitate obtained from glue by tincture of gall- 
nuts, therefore glue, when dissolved in acetic acid, will not be 
precipitated by the tincture.) According to these reactions, 
the gelatinous substance obtained appears to be chondrin, not- 
withstanding that it was obtained from ossified cartilage. The 
question, therefore, arises—does the cartilaginous substance 
which is connected with earthy matter in the fetus really 
yield chondrin instead of the gelatine of bone, or was there 
much unossified cartilage still contamed in what appeared to 
be ossified, and was that the sole source of the chondrin? The 
point is, at all events, worthy of renewed investigation. It is 
surprising that the foetal cartilages should exhibit so great a 
resistance to the action of boiling water, and that although 
they yield a small quantity of a gelatinous substance, they do not 
afford any which has the property of gelatinizing. 

The formative processes of cartilage hitherto described, 
proceed, as it appears, without the presence of vessels in the 
structure ; such at least is the case in thin cartilages, to which 
probably the fiuid parts of the blood can penetrate from the 
vessels of the neighbouring tissues. In the branchial rays of 
the fish, for example, I could not find any space in which ves- 
sels could have existed; throughout the structure masses of 
cartilage and cartilage-corpuscles were to be seen, but no canals 
which could have been traversed by vessels. 

The manner in which ossification proceeds now becomes an 
interesting object of inquiry. The investigation is best pur- 
sued by making very fine sections with a razor, from the half- 
ossified cartilages of the extremities, vertebrae, or coccyx, of 
the larva of Pelobates fuscus. ‘The little cartilage-cells, which 


28 STRUCTURE AND GROWTH 


are not enclosed one within another, and are for the most part 
furnished with a nucleus, are readily recognized in the true 
cartilaginous substance of the unossified cartilages. I am not 
prepared to state whether this substance is formed by thicken- 
ing of the cell-walls, or by the intercellular substance. The 
earthy matter is first deposited in the true cartilagmous sub- 
stance. It first appears in the form of isolated, extremely 
minute granules, by which an indistinct appearance of arched 
striee is sometimes produced. At other points, these little gra- 
nules of earthy matter lie collected together into larger irregu- 
lar heaps. I do not know whether these little collections are 
depositions of pure earthy matter which has not as yet united 
with the cartilage, and therefore merely provisional deposits 
which subsequently are distributed equally in the cartilaginous 
substance (which is not probable), or whether this earthy 
matter is already united with the cartilage, and that the regular 
aspect which the structure presents when ossified may be ac- 
counted for by the gradual union of the earthy matter with it 
after the same mode. I saw no such deposition of earthy matter 
in heaps in the incompletely ossified parietal bones of the same 
larva, but the whole cartilaginous substance contained it equably 
distributed without any perceptible granules. In both instances, 
however, when dilute hydrochloric acid is applied to the object 
under the microscope, the boundary denoting the solution of the 
earthy matter, and the consequent transparency of the cartilage, 
may be distinctly seen advancing in the form of a sharply-defined 
line from the edge of the preparation towards the interior, proving 
that, in the cartilages first mentioned, there was earthy matter 
equably united with the substance, in addition to the heaps and 
isolated granulous deposits. For this boundary line cannot be 
produced by the mere progressive imbibition of the acid with- 
out a solution of the earthy salts; at least neither an unossi- 
fied cartilage, nor one from which the earthy matter had been 
previously withdrawn and the acid again washed from it, ex- 
hibited the phenomenon of such a line advancing towards the 
interior. During the early period of ossification, when this 
line arrives at a cell-cavity, it becomes indented proportionally 
to the size of the cavity, because it does not come in contact 
with any earthy matter there; the cell-cavities, in the first 
instance, being free from earthy salts. The reverse, however, is 


OF CARTILAGE. 29 


the case in the more completely ossified parts; there the cell- 
cavity remains behind, forming a dark indentation in the 
line, which as it advances renders the tissue transparent, and 
leaves the cavity a black spot, from which dark fibres, simi- 
lar to those of the corpuscles of bone, issue in a stellated form. 
Shortly afterwards the fibres disappear, then the corpuscle gra- 
dually diminishes, and at last vanishes also, leaving a pale spot. 
Such an appearance could not be due to an air-bubble in the 
cell-cavity ; for in that case, I think, the course of its exit 
might be followed. It is probably a more compact mass of 
earthy matter, which does not become dissolved so quickly as 
that contained in the substance of the cartilage. After this 
has become impregnated with earthy matter, the cell-cavities are 
also filled, and when so filled they are the osseous corpuscles. 
Similar observations might be instituted on the ossified carti- 
lages of mammalia, in which the identity of osseous and carti- 
lage-corpuscles was rendered more certain by Miescher’s re- 
searches. The next question which presents itself concerns 
the nature of those minute fibres which proceed in a stel- 
lated form from the osseous corpuscles. After the earthy mat- 
ter has been withdrawn the corpuscles may still be seen, though 
rendered very pale by that process; the fibres, however, are 
not at all visible, although a formation corresponding to them 
is certainly present in the cartilaginous substance, and their 
extraordinary minuteness sufficiently explains the invisibility. 
The same formation might also exist before ossification, but 
be invisible from the like cause. As these fibres and the 
cell-cavities become filled with earthy matter simultaneously, 
and at a later period than the cartilaginous substance, and 
since they contain the earthy salts in a more compact and 
less easily soluble mass, it is probable that they are hollow 
tubes, that is, canaliculi which proceed from the cell-cavities, 
spreading out into the cartilagmous substance. According, 
therefore, to the view which we take respecting the cartilage- 
corpuscles, according as we consider them to be the cavities of 
cells, the walls of which have become thickened and blended, not 
only with one another but with the intercellular substance, so 
as to form the cartilaginous substance; or as we take them 
for the entire cells, and the intermediate substance of the 
cell-cavities as only intercellular substance, so must these tubes 


30 STRUCTURE AND GROWTH 


be viewed either as canaliculi which penetrate from the cell-cavity 
into the thickened cell-walls, or as hollow prolongations of the 
cells into the intercellular substance. In the first case, they 
might be compared to the porous canals of vegetable cells; in 
the second, they would correspond with prolongations of 
cells, such as we shall often again meet with in the progress of 
this work. Meanwhile, for an example of those cells which 
are extended out on all sides into canals, and which I have 
called stellated cells, the reader is referred to plate LI, figs. 8 
and 9, where those transformations are delineated from pigment- 
cells. I decidedly give the preference to the latter explanation 
of the canaliculi, because they pass through the entire thick- 
ness of the firm cartilaginous substance, a fact which, in order 
to be consistent with the first view, requires for its explanation 
that the substance between the cell-cavities should be formed 
of the thickened cell-walls, which is certainly not the case 
in the cartilages of mammalia, as is seen in plate III, fig. 2. 
The osseous corpuscles, with their canaliculi, would therefore 
be the cartilage-cells transformed into stellated cells, and filled 
with earthy matter. We shall return to this metamorphosis 
of round into stellated cells when treating of the pigment. The 
resemblance between stellated pigment-cells and osseous cor- 
puscles is sometimes very striking, as is shown, for example, 
by the pigment-cell which hes to the extreme right in plate II, 
fig. 9. The compact bony substance is intercellular substance ; 
it is, however, probable that the walls of the stellated osseous 
cells form some, if only a very small part, of it. 

When ossification takes place, the earthy matter is first de- 
posited in this intercellular substance, and probably at a sub- 
sequent period also in the cell-cavities. The deposition often 
causes the substance to assume a darkish granulous appearance 
in the first stance, which it afterwards loses, becoming more 
equally dark. If we assume, what is extremely probable, that 
the earthy matter is contained in bones in combination with 
the cartilaginous substance, in a manner analogous to a che- 
mical union, and not in the form of minutely-divided granules, 
the mode in which the union with the earthy salts takes place 
may then be explained in two ways: either the earthy matter 
combines with a particle of cartilaginous substance in such a 
manner that each smallest atom receives in the first instance a 


OF CARTILAGE. 31 


minimum of salts, and gradually more and more, until the whole 
portion of cartilage obtains its due quantity; or, the earthy 
matter unites at first with some only of the smallest atoms of 
the cartilage, combining, however, with these to the full propor- 
tion which their capacity of saturation requires ; the remaining. 
atoms then gradually and successively receive their due portion 
of the salts, so that each atom does not chemically combine with 
them until it can become completely saturated. The latter 
explanation, from the analogy with organic combinations, and 
from the above-mentioned granulous appearance which cartilage 
exhibits when undergoing ossification, appears to me by far the 
moreprobable. For, according to the first view, the medullary ca- 
naliculi, in the neighbourhood of which the deposition of earthy 
matter first commences, ought to be surrounded, not by a gra- 
nulous appearance, but by a dark shadow which should gradu- 
ally fade away to a pale edge. 

I conceive the formation of the medullary canaliculi in ossi- 
fying cartilage to be similar to that of the capillary vessels, 
which will be examined hereafter. We shall return to them 
again, as also to the origin of the concentric laminz of bone. 

We will now briefly sum up the observations upon cartilage, 
and refer to the phenomena of vegetable life, which either accord 
with or are dissimilar to them. Cartilage originates from cells, 
every one of which has its special, and, in the first instance, 
very thin wall; precisely like those of vegetables. These cells 
either lie closely together, and on that account are flattened 
against one another, like those of plants (see pl. I, figs. 5 and 6), 
or, there is intercellular substance present, and this again either 
in so very small a quantity as to be visible only in situations 
where three or four cells are in contact (see fig. 6, c), or in 
so much greater quantity, as to prevent the contiguity of the 
different cell-walls (pl. I, fig. 7; and pl. ITI, fig. 1.) Most 
of the cells, at their earliest period of development. (and _per- 
haps constantly) contain a nucleus, that is, a round or oval, and 
sometimes hollow corpuscle (pl. I, fig.5, @; and pl. ILI, figs. 1 
and 2), which again generally encloses one or two nucleoli. 
The cartilage-cells originate in the first place by the formation 
of the nucleus in the cytoblastema, around which the cell is 
afterwards formed, so that the latter at first closely encompasses 
the nucleus. The nucleus advances slightly in growth after the 


32 STRUCTURE AND GROWTH 


formation of the cell, but in a much lower proportion. It is 
subsequently absorbed ; frequently, however, not before ossifi- 
cation. This is precisely what occurs in vegetables. The walls 
of the cartilage-cells become thickened (compare figs. 6 and 7 
with fig. 5), which is also the case with many vegetable-cells. 
No distinction, however, between primary cell-membrane and 
secondary deposit can be observed in cartilage-cells, and such a 
deposition in strata as is often distinctly seen in thickened cells 
of plants cannot be made out here with sufficient certainty. 
The cell-nucleus in the meantime, when not absorbed, remains 
lying upon the inside of the thickened wall. An instance of 
actual thickening of the cell-membrane without a stratified 
deposit, does not, however, appear to be wanting in plants, 
e.g. the pollen-tube of Phormium tenax. (See the Introduction.) 
But it seems, that a thickening of the walls of the cartilage- 
cells does not take place universally, it does not for instance in 
the ossifying cartilages; the true cartilage substance may also 
be formed entirely, or at least chiefly of the intercellular sub- 
stance. The condensed cell-walls subsequently coalesce with 
one another, or with the intercellular substance, so that at last 
only the cell-cavities remain in an homogeneous substance. 
Whether the walls of those cartilage-cells which do not undergo 
any thickening become blended with the intercellular substance 
or not, remains uncertain. An analogous instance of coalescenec 
of the cell-walls is afforded by vegetables, for Schleiden has ob- 
served such a blending in the layer of bark which lies im- 
mediately beneath the cuticle of the Cacti. 

The cartilage-cells often contain either simple nuclei (i. e. 
without cells around them), or young cells with such nuclei. 
These young cells are formed free within the parent-cell, 
without vascular connexion. Their nucleus is first formed, and 
afterwards the cell around it, just as in the true cartilage-cell. 
This is one of the most important instances of accordance be- 
tween animal and vegetable cells, for the latter, according to 
Schleiden, are developed in like manner from the nucleus, and 
likewise within a parent-cell. (See the Introduction.) We may 
therefore confidently compare the nucleus of these young cells, 
as also that of the true cartilage-cell, to the cytoblast of vege- 
table cells. Their shape and the eccentric position of their 
nucleus, placed as it is upon the internal surface of the cell-wall, 


OF CARTILAGE. 33 


also accord with the young cells of plants. Compare plate I, 
fig. 8, ff, with fig. 2. The form of the nucleus likewise corre- 
sponds with that of many vegetable cells. In these young cells 
of cartilage, it is presented to the observer as a small oval or 
perfectly spherical corpuscle, having, in many instances, a 
granulous and somewhat yellowish appearance, and containing 
one or two nucleoli. (Compare this with the description of the 
nucleus of vegetable cells in the Introduction.) The nucleus of 
the cartilage-cell appears to be hollow, a fact which has not 
been observed with regard to the cytoblast of vegetable cells,’ 
and the nucleoli lie close upon, or in the neighbourhood of the 
internal surface of its wall, whilst, according to Schleiden, they 
lie deep in the cytoblast of vegetable cells. 

The cartilage-cells, when once formed, appear to be endued 
with the capacity to grow throughout the entire mass of the 
structure. The case is different with regard to the formation 
of new cells. This takes place in certain situations only, on 
the surface of the cartilage, for imstance, or between the last 
formed cells. We have already scen that in the branchial rays 
of fishes, the least developed cells lay at the point, and 
lateral margins. The little rod, which the branchial ray 
represents, does not increase in size by the formation of new 
cells between the original ones throughout its entire length, 
but its extension in the longitudinal direction is produced 
by the development of new cells in the neighbourhood of 
the poimt, and it increases in breadth by the same process 
going on in the neighbourhood of the side walls. It is a 
familiar fact, that the cylindrical bones grow chiefly upon the 
surface and at the end of the shaft. The formation of new 
cartilage-cells usually takes place only in the neighbourhood of 
the surface which is in contact with the organized substance, 
(I refer throughout this passage to that period alone, at which 
the cartilage does not contain any vessels of its own,) but it 
is not exclusively confined to that situation, it may also 
proceed in the intercellular substance between the last-formed 
cells. 

At the period of ossification, the earthy matter is first de- 
posited in the cell-walls, or in the true cartilage-substance, the 


' In a letter which I have received from Schleiden, he informs me that he has 
also found hollow nuclei in plants. 


3 


34 STRUCTURE AND GROWTH 


remains of the cell-cavities also become filled with it at a 
subsequent period, and at the same time the stellated canali- 
culi issuing from them make their appearance. The formation 
of these canaliculi probably takes place by the transformation 
of round cartilage-cells into a stellated form, after the manner 
of the pigment-cells at plate II, figs. 8 and 9. 


The above detailed investigation of the chorda dorsalis and 
cartilage, has conducted us to this result,—that the most mmpor- 
tant phenomena of their structure and development accord with 
corresponding processes in plants, that some anomalies and 
differences may indeed still remain unexplained, but that 
they are not of sufficient importance to disturb the main con- 
clusion, viz. that these tissues originate from cells, which 
must be considered to correspond im every respect to the 
elementary cells of vegetables. Thus then are we furnished 
with the first of the proofs required in the Introduction ; that 
is to say, we have shown with regard to a certain tissue, that 
it not only origimates from cells, but that these cells in the 
process of their development manifest phenomena analogous to 
those of the cells of plants. We have now thrown down a 
grand barrier of separation between the animal and vegetable 
kingdoms, viz. diversity of structure. We have become ac- 
quainted with the signification of the individual parts of the ani- 
mal tissues as compared with the vegetable cells, and know that 
cells, cell-membrane, cell-contents, nuclei, and nucleoli in the 
former are in every respect analogous to the parts having 
similar names in the cells of plants. We have already observed 
several modifications both of the nucleus and cell. The former 
presented itself as a corpuscle having either an oval or circular 
outline, spherical in figure, or very much flattened, sometimes 
hollow, and often scarcely perceptible, in consequence of its 
transparency, but generally granulous and yellowish, and con- 
taining in its imterior from one to three nucleoli. This 
nucleus lay within, and fast adhering to the wall of the cell, 
but never in its centre. The fundamental form of the cell 
appeared to be that of a round vesicle, but we have also ob- 
served the flattening of the cells agaimst one another, the 
presence of intercellular substance between them in greater 
or less quantity, and lastly, the thickening of the cell-walls. 


OF CARTILAGE. 35 


We have seen the generation of cells within cells, and the 
formation both of the young cells in cartilage, and of the 
true cartilage-cells themselves, was proved to take place 
around the nucleus, in the same manner as that described 
by Schleiden in vegetable cells. The other proof for the 
accordance of animal and vegetable structure (see Introduction, 
p- 6) yet remains to be supplied, viz. that most or all animal 
tissues are developed from cells. If this proof only were 
furnished, the analogy of such cells to the elementary cells of 
plants would at once become extremely probable ; we may now 
assert that analogy so much the more firmly, since the cells 
of two distinct tissues have been proved in detail to correspond 
with those of plants. 


SECTION II. 
ON CELLS AS THE BASIS OF ALL TISSUES OF THE ANIMAL BODY. 


Tue young cells contained within the cartilage-cells (see 
plate I, fig. 8, ff) may be regarded as the elementary form 
of the tissues previously considered, and may be described as 
round cells having a characteristic nucleus, firmly attached to 
the internal surface of the wall. As the above were proved to 
correspond with the vegetable cells, it follows, that it is only 
necessary to trace back the elementary structure of the rest 
of the tissues to the same formation, in order to show their 
analogy also with the cells of plants. In some tissues this 
proof is easy, and immediately afforded ; in others, however, it 
is obtained with much difficulty, and it would frequently be 
altogether impossible to demonstrate the cellular nature of 
some, if the connexion between the different steps in this 
investigation were lost sight of. The difficulty arises from the 
following circumstances: Ist. The minuteness of the cells; in 
consequence of which it is not only necessary to use a power 
magnifying from 400 to 500 diameters, but it is also frequently, 
indeed generally found impossible to press out their contents. 
2dly. The delicate nature of the cell-membrane. When this has 
a certain density, its external as well as internal outlmé may 
be recognized, and the distinction between it and the cell-con- 
tents may thus be placed beyond a doubt. Butif the cell-mem- 
brane be very delicate, the two outlines meet together im one line, 
and this may readily be regarded as the boundary line of a 
globule, not enclosed by a special enveloping membrane. 3dly. 
The similar power of refraction possessed by the cell-wall and 
cell-contents, in consequence of which the internal outline 
of the former cannot be observed. 4thly. The granulous nature of 
the cell-membrane, which when the contents are also granulous, 
cannot be distinguished from them. Lastly, the variety of 


ON CELLS, ETC. oT 


form presented by the cells, for they may be flattened even to 
the total disappearance of the cavity, or elongated into cylinders 
and fibres. From these circumstances, many of the cells which 
now come before us for consideration, have been described as 
mere globules, or granules, terms which do not express their 
true signification, and even when they were spoken of as cells, 
or cells furnished with a nucleus, the description rested only 
upon a slight analogy, since but very few of them (for example, 
the pigment-cells), were proved to be actually hollow cells. 
But—as the precise signification of the nucleus is unknown, and 
as the celi-membrane is not proved to be anything essential to 
those cells (and this follows from their accordance with vege- 
table cells), upon the analogy with which the proof of the 
cellular nature of the rest of the globules provided with a 
nucleus will be based,—there is no contradiction involved in the 
supposition that a nucleus may be contained in a solid globule 
as well as in a cell. 

From the difficulties of this investigation above detailed, it 
will be seen that a given object may really be a cell, when even 
the common characteristics of that structure, namely, the per- 
ceptibility of the cell-membrane, and the flowing out of the cell- 
contents, cannot be brought under observation. The possibility 
that an object may be a cell, does not, however, advance us 
much; the presence of positive characteristics 1s necessary in 
order to enable us to regard it as such. In many instances 
these difficulties do not present themselves, and the cellular 
nature of the object is immediately recognized ; in others, the 
impediments are not so great but that the distinction between 
cell-membrane and cell-contents is at least indicated, and in 
such cases other circumstances may advance that supposition 
to a certainty. The most important and abundant proof as to 
the existence of a cell is the presence or absence of the nucleus. 
Its sharp outline and dark colour render it in most instances 
easily perceptible ; its characteristic figure, especially when it 
encloses nucleoli, and remarkable position in the globule under 
examination, (being within it, but eccentrical, and separated 
from the surface only by the thickness of the assumed cell-wail,) 
all combine to prove it the cell-nucleus, and render its analogy 
with the nucleus of the young cells contained in cartilage, and 
with those of vegetables, as also the analogy between- the 


38 ON CELLS AS THE BASIS 


globules under examination, in which it lies, and those cells, 
consequently the existence of a spherical cell-membrane in the 
globules, extremely probable. More than nine tenths of the 
globules in question present such a nucleus; in many the 
special cell-membrane is indubitable, in most it is more or 
less distinct. Under such circumstances, we may be permitted 
to conclude that all those globules which present a nucleus of 
the characteristic form and position, have also a cell-membrane, 
although, from the causes before specified, it may not be per- 
ceptible. The different tissues will also afford us many instances 
of other circumstances which tend to prove the existence of 
an actual cell-membrane. An example of what is referred to 
would be afforded by an instance, in which a certain corpuscle 
(furnished with a nucleus), about the cellular nature of which 
a doubt existed, could be proved to be only a stage of deve- 
lopment, or modification in form, of an indubitable cell. The 
cell-nuclei and their distance from each other when scattered 
in a tissue, also serve as indications, when the outlines of the 
cells have to be sought for. They likewise serve to guide 
conjecture as to the earlier existence of separate cells, mm 
instances where they have coalesced in the progress of develop- 
ment. When a globule does not exhibit a nucleus during 
any one of the stages of its development, it is either not a cell, 
or may at least be preliminarily rejected, if there be no other 
circumstances to prove it such.’ Fortunately, these cells devoid 
of nuclei are rare. 

In addition, however, to the cellular nature of the elementary 
structures of animal tissues, there are yet other points of 
accordance between them and the cells of plants, which may 
generally be shown in the progress of their development, and 
which give increased weight to the evidence tending to prove 
that these elementary structures are cells. The exceedingly 
frequent, if not absolutely universal presence of the nucleus, 
even in the latest formed cells, proves its great importance for 
their existence. We cannot, it is true, at present assert 
that, with regard to all cells furnished with a nucleus, the 
latter is universally the primary and the cell the secondary 
formation, that is to say, that in every instance the cell is 
formed around the previously existing nucleus. It is probable, 
however, that such is the case generally, for we not only meet 


OF ALL ANIMAL TISSUES. 39 


with separate nuclei in most of the tissues, distinct from those 
which have cells around them, but we also find that the 
younger the cells are, the smaller they are in proportion to 
the nucleus. The ultimate destiny also of the nucleus is 
similar to that of the vegetable cells. As in the last named, 
so in most animal cells it is subsequently absorbed, and remains 
as a permanent structure in some few only. In plants, ac- 
cording to Schleiden, the young cells are always developed 
within parent cells, and we have also seen such a development of 
new cells within those already formed in the chorda dorsalis 
and cartilage. If, however, any doubt existed as to whether 
the primary cells of these tissues were formed within previously 
existing parent cells, none such can arise in reference to many 
of the tissues next to be considered. We shall indeed fre- 
quently meet with a formation of young cells within older 
ones, but it is not the rule, and does not occur at all with 
regard to many of them. 

The following admits of universal application to the forma- 
tion of cells; there is, in the first instance, a structureless! 
substance present, which is sometimes quite fluid, at others 
more or less gelatinous. This substance possesses within 
itself, in a greater or lesser measure according to its 
chemical qualities and the degree of its vitality, a capacity to 
occasion the production of cells. When this takes place the 
nucleus usually appears to be formed first, and then the cell 
around it. The formation of cells bears the same relation to 
organic nature that crystallization does to inorganic. The 
cell, when once formed, continues to grow by its own individual 
powers, but is at the same time directed by the influence of 
the entire organism in such manner, as the design of the 
whole requires. ‘This is the fundamental phenomenon of all 
animal and vegetable vegetation. It is alike equally consistent 
with those instances in which young cells are formed within 
parent cells, as with those in which the formation goes on 


' [Strukturlos.—I have ventured to translate this word as above, although I am 
aware it is open to objection. The idea intended to be conveyed by the author is 
that of a substance in which no definite structure can be detected. As the word 
will be frequently used in the following pages, the reader is requested to assign this 
signification to it invariably— TRANS. ] 


40) THE OVUM AND 


outside of them. The generation of the cells takes place in a 
fluid, or in a structureless substance in both cases. We will 
name this substance in which the cells are formed, cell-germi- 
nating material (Zellenkeimstoff), or cytoblastema. It may 
be figuratively, but only figuratively, compared to the mother-lye 
from which crystals are deposited. 

We shall refer to this point at greater length hereafter, and 
only anticipate our subject with this result of the investigation, 
in order to facilitate the comprehension of what follows. 

In the previous section of this work we have discussed in 
detail the course of development of some of the animal cells, 
having taken the chorda dorsalis and cartilage for our examples. 
We are now required to prove, as far as is possible, that all 
the tissues either originate from, or consist of cells. We 
separate this investigation into two divisions. The first treats 
of the Ovum and Germinal membrane, in so far as they form 
the common basis of all the subsequent tissues. The second 
division embraces the permanent tissues of the animal body, 
with the omission of the two already described. 


FIRST DIVISION. 


On the Ovum and Germinal Membrane. 


The ovum of Mammalia lies, as is known, within the Graafian 
vesicle. J have not made any investigation as to whether that 
vesicle may be considered to have the signification of a cell. 
It is deed a cell in the general sense of the word, being a 
cavity in the substance of the ovary, it has even a special 
membrane ; but as we here only receive the word cell as sig- 
nifying an elementary part of animals and plants, it becomes 
necessary to inquire whether this membrane may not be a 
secondary formation resulting from the junction of other struc- 
tures which are elementary. The history of the development 
of the Graafian vesicle must show whether that be the case, or 
whether it originate by the mere growth of a cell furnished 
with a structureless cell-membrane, which cell may formerly, 


GERMINAL MEMBRANE. 41 


perhaps, have had a nucleus.’ Within this vesicle lies the 
ovum or vesicle of Baer, embedded in a layer of granules. 
When these granules are examined with a magnifymg power 
of 450, they are readily recognized to be cells, that is, round 
vesicles containing a nucleus, which is situated upon the 
internal surface of the wall. The nucleus being granulous 
and darker than the rest of the object falls under observation 
first. It encloses one or two nucleoli. The cell surrounding 
it varies in size, being in the average about half as large again 
in diameter, but some are much larger. The cells are for the 
most part extremely delicate, and round, when separated from 
one another. When in connexion, they often flatten against 
one another, and assume a polyhedral form. In addition to 
these cells, isolated nuclei appear also to be present within the 
Graafian vesicle, perhaps as the germs of new cells. The pro- 
duction of these cells proceeds according to the fundamental 
law mentioned at page 39, within the fluid of the Graafian 
vesicle, that being their germinative material or cytoblastema. 
Whether this fluid is to be regarded as cell-contents, and the 
cells produced in it as being formed within a parent cell, must 
depend upon the solution of the question, as to whether the 
Graafian vesicle be an elementary cell or not; but the deci- 
sion of this point is not essential, for the rule that cells 
originate within others is not universal. When the inde- 
pendent vitality of cells is borne in mind, we can readily 
conceive how these, when they (after the bursting of the 
vesicle) arrive with the ovum in the uterus, may be further 
developed into other structures (the chorion according to 
Krause.) Within this granulous or rather cellular disc then 
the ovum or vesicle of Baer lies embedded, (see the represen- 
tation, plate II, fig. 1, taken from Krause.) The first object 
which attracts observation is the dark spherical yelk, surrounded 
by a transparent space, (zona pellucida of Baer, chorion of 
Wagner.) Krause found (Miller’s Archiv, 1837, p. 27) that 
the yelk is surrounded by a peculiar membrane, d (vitelline 
membrane), and that the transparent space is enclosed externally 


' According to the researches of Martin Barry (Phil. Trans. Part II, 1838, p. 305, 
&c.), both cases appear to occur, so that a cell composed of a structureless mem- 
brane is first formed, (the ovisac of Barry,) and subsequently an external vascular 
covering of cellular tissue. On the relation of this follicle to the mode of develop- 
ment of the oyary itself, see Valentin in Miiller’s Archiv, 1838, p. 526. 


42 THE OVUM AND 


by a very delicate pellicle, the albumen-membrane, 6, also that 
the transparent substance itself (albumen) is sufficiently fluid to 
permit of such a degree of displacement of the yelk as to allow 
of its coming into contact even with the albumen-membrane. 
Although I have never yet succeeded in observing this pellicle, 
and though in my researches the transparent membrane, on 
the bursting of the yelk, always tore with smooth edges like a 
solid substance, yet the observations of the respected discoverer 
are too precise to admit of a doubt upon it. It is also sup- 
ported by the analogy of most of the ova of other classes of 
animals, in which chorion and vitellme membrane may gene- 
rally be distinguished, notwithstanding that they sometimes lie 
close upon each other. The albumen-membrane has probably 
the signification of a cell-membrane, in which case the albumen 
will be the cell-contents, and the yelk a young cell. Accord- 
ing to Wharton Jones, the transparent areola (zona pellucida) 
of the ovum, or the albuminous layer in the fecundated 
ovum of mammalia, becomes considerably expanded in the 
tubes, a fact which would be readily explained by the inherent 
energy of the albumen-membrane when regarded as a cell. 
In such case, however, the mode of formation of the albumen 
would be very different from the corresponding process in the 
bird’s egg, where, according to Purkinje, it is secreted by the 
oviduct, and a membrane (chorion) is formed around it sub- 
sequently, which cannot therefore have the signification of a 
cell-membrane, and is moreover not simple in structure, but 
composed of fibres. Meanwhile an investigation might be 
made, as to whether the albumen in the egg may not also be 
first surrounded and formed by an equally thin pellicle, 
around which a secondary external membrane may subsequently 
be produced. According to Purkinje, however, this is not 
the case, and I could not discover any such pellicle upon the 
inner surface of the shell-membrane of the excluded egg. I 
have not made any inquiry as to whether the chorion of fishes 
is a cell-membrane or not. It is covered internally with a 
very beautiful epithelium, which is made up of more or less 
flat hexagonal cells, each of which has its nucleus. 

Within the transparent areola, or, according to Krause, the 
albuminous layer, lies the vesicle of Baer, or the yelk ; which, 
from Krause’s statement, is enclosed by a peculiar structureless 


GERMINAL MEMBRANE. 43 


membrane, the double outline of which he recognised, (plate 
II, fig. 1, d.) It is thus highly probable that the yelk of the 
mammalian ovum is a cell. Even if, as Wagner intimates, the 
vitellime membrane in other animals should sometimes be 
formed only secondarily within the chorion, it would not 
materially interfere with our purpose, since in that case the 
chorion would be the cell-membrane. ‘The ovum universally 
possesses an external closed membrane (whether it be chorion 
or vitelline membrane), which is structureless, and not gene- 
rated from other elementary structures, and therefore is the 
ovum always a cell. The yelk-cell encloses the vitelline sub- 
stance as its cell-contents, and upon its internal surface lies 
the germinal vesicle, or vesicle of Purkinje, (fig. 1, /) 
This, as is known, is a very transparent thin-walled vesicle, 
containing a pellucid fluid, according to Wagner coagulable by 
spirits of wine. It encloses almost universally (Wagner cites 
but very few exceptions) upon the internal surface of its wall, 
a corpuscle, called by the discoverer, R. Wagner, germinal spot, 
or germinal disc, (fig. 1, g.) Im mammalia it is generally flat. 
In many instances several of these spots are present, their 
number, however, is said by Wagner to bear proportion to the 
age of the ovum, they beg fewer and much more firmly 
attached to the wall of the germinal vesicle in young ova, I have 
frequently observed in osseous fishes (where they are often pre- 
sent in such numbers as to prevent the fluid in the vesicle from 
being seen) that when one of these corpuscles, after the bursting 
of the germ-vesicle, passed through a narrow space, it first 
became considerably elongated, and then drawn out in the 
centre to a thin thread, which soon broke. The two ends 
afterwards retracted, and thus two round globules were pro- 
duced from one corpuscle, in a similar manner to what we may 
observe in the drops of fat upon soup. They appear, therefore, 
to be composed of a tenacious substance which is not miscible 
with water. Purkinje states that the germinal vesicle in birds 
is firmly fixed to the vitellme membrane, but Baer and 
Wagner describe it as lying in the centre of the yelk at first, 
and rising to the surface at a subsequent period. 

The decision of the question, as to the precise signification 
of the germinal vesicle, now becomes of great importance. Is 
it a young cell generated within the yelk-cell, or is it the 


da THE OVUM AND 


nucleus of the yelk-cell? If the former, it is in all probability 
the most essential rudiment of the embryo ; but if it be the 
nucleus of the yelk-cell its importance vanishes with the forma- 
tion of the yelk-cell, and according to the analogy of most 
cell-nuclei, it must either become absorbed altogether at a 
subsequent period, or continue for a time simply rudimentary, 
without forming any important new structure. The follow- 
ing is the ordinary career of a simple cell: a nucleus is 
present in the first instance; around it a cell is formed ; the 
nucleus at first often increases in size as the cell grows, but 
their growth is by no means proportionate, that of the cell 
being much more rapid ; the cell-contents are at first transpa- 
rent ; a firm precipitate or new formation next commences in 
the cell, and this occurs immediately around the nucleus, 
which is at first enclosed by it; the nucleus then either 
becomes entirely absorbed, or continues only rudimentary and 
(with the following exception) I have never observed it to 
give origin to any other essential formation. One or more 
oil-globules once appeared to me to be formed during the ab- 
sorption of the nucleus in the adipose cells within the cranial 
cavity of a young carp. The importance of the decision of 
this question in reference to the germinal vesicle thus becomes 
very obvious. Unfortunately, however, neither the observa- 
tious upon the subsequent relations of the germ-vesicle, nor 
those on the origination of the ovum, are sufficiently extensive 
or certain for the purpose. 

We shall next proceed to analyse both views of the question 
more minutely, and afterwards compare them with the obser- 
vations. If the germ-vesicle be a young cell, in the first place, 
it is absolutely necessary that the yelk-cell should first exist, 
and that the germ-vesicle should afterwards be developed within 
it; 2dly, the germ-vesicle must not be connected with the 
vitelline-membrane, but must be developed free at some chosen 
spot within the cavity of the yelk; 3dly, the germ-vesicle 
may be regarded either as a cell without a nucleus, and in 
that case the spots of Wagner belong to the cell-contents, or 
Wagner’s spot, when it is single, is the nucleus; when there 
are several present, the others either differ essentially from one 
particular spot, and pertain to the cell-contents, or they are 
nuclei of young cells afterwards to be developed within the 


GERMINAL MEMBRANE. 45 


germ-vesicle. Before the spot can be considered to be the nu- 
cleus, it is necessary that it should, in the first instance at 
least, be connected with the wall of the vesicle. If, however, 
the germinal vesicle be the nucleus of the yelk-cell, it is 
essential, in the first place, that it should, in all probability, be 
present before the yelk-cell; at all events, that in proportion 
as the ovum is younger, should the vesicle be larger in relation 
to the cell; 2dly, it must, at first, he upon the vitelline- 
membrane, and be more or less intimately connected with it ; 
ddly, the germinal-vesicle, when regarded as a nucleus, either 
has no nucleoli, or Wagner’s spots are to be considered to re- 
present them; in the first case they form the contents of the 
nucleus. In the enumeration of these poimts, no regard is 
had to the relations of the germ-vesicle subsequent to impreg- 
nation, because it is desirable to determine its ultimate destiny, 
to a certain extent a priori, from its signification, and thus to 
be enabled at the least to afford a guide to the much moré 
difficult observation of the fecundated ovum. If the researches 
were complete, the distinctions above cited would be sufficient 
for the correct determination of the question at issue, the 
decision of the first pomt imdeed would of itseif be ample 
evidence. 

When we take into consideration the first point raised on 
either side, we should be compelled to decide in favour of the 
latter view, and regard the germ-vesicle as a nucleus, if it were 
proved to be first present, and also that the yelk-cell is formed 
around it as a simple cell, narrowly encompassing it in the 
first instance, and becoming gradually expanded. In the next 
place, it is certain that at an early period the germ-vesicle 
is much larger in proportion to the yelk-cell, and that it 
at first grows part passu with the yelk-cell, but that subse- 
quently the latter mcreases in size in a much greater ratio, 
whilst the vesicle remains stationary; and these are precisely 
the relations in which the vesicle should stand in order to be 
regarded as a nucleus. But these facts are not entirely irre- 
concilable with the first view. A young cell, the germ-vesicle, 
might be imagined to form within the yelk-cell at a very early 
period of its growth, which young cell might at first increase 
in size more rapidly than the original one, but cease to do so 
earlier, whilst the parent-cell might continue to be developed 


46 THE OVUM AND 


in size. Such a circumstance is, however, very rare, and the 
weight of evidence before us is much in favour of the second 
view ; but in order to determine this point, it is necessary to 
inquire whether the vesicle exist before the cell. That such 
is the case is not yet proved, although Baer and Purkinje sup- 
pose it to be so, and an observation of Wagner’s favours the 
supposition. (Prodromus Physiologiz Generationis, p. 9, fig. 
xvii, a.) He found the posterior extremity of the oviduct of 
Acheta campestris full of germinal vesicles, which became gra- 
dually expanded in their progress through the oviduct. The 
oviduct becomes dilated in its further course; globules are 
observed in it, which Wagner regards as yelk-globules, and 
between them lie the germ-vesicles; then “each vesicle becomes 
surrounded by its yelk and chorion, and thus the individual 
ova become separated.” He does not state, however, in what 
manner the vitelline-membrane is produced. Is it formed as 
a cell, at first narrowly encompassing the germ-vesicle, and 
then gradually expanding; or does it at the same time enclose 
a quantity of the surrounding yelk-globules? It is difficult 
to conceive the latter mode of formation ; but if the former be 
the correct one, the globules surrounding the germ-vesicles in 
the oviduct cannot be yelk-globules. Fresh researches are 
therefore necessary, which, if they should be confirmatory of 
the first view, will also be decisive for considering the germ- 
vesicle as a cell-nucleus.' 

With regard to the second point,—namely, as to whether the 
germ-vesicle be more or less intimately connected with the 
membrane of the yelk-cell at an early period, or lie free within 
it,—any evidence afforded by its solution would be comparatively 
inconclusive. According to Baer and Wagner, the vesicle in 
the first instance lies in the centre of the yelk-cell, and only 
rises to its wall at a later period. Baer quotes the ova of 
frogs as examples in which it lies for a long time in the centre 
of the yelk. The germ-vesicle is generally found on the wall 
of the cell; and in birds, according to Purkinje, it is frequently 
so intimately connected with it, that it tears im the attempt to 


' See the Supplement. The observations of Wagner upon the ova of insects 
which are there quoted, and the recent researches of Barry on those of mammalia 
and birds, (1. c. p. 308,) prove the germinal vesicle to be first formed, and then the 
vitelline membrane round it. 


GERMINAL MEMBRANE. 47 


separate them. Although the position of the vesicle in the 
middle of the yelk-cell affords evidence rather in favour of its 
being regarded as a young cell, yet it is not altogether incon- 
sistent with its character as a nucleus; for it is only during 
the earliest formation of the cell that the nucleus is required 
to be connected with it; it is frequently disconnected at a 
later period, and lies loose in the cell. At that stage of deve- 
lopment, however, im which the vitelline-membrane closely en- 
compasses the germ-vesicle, it is impossible to decide whether 
it he in the middle or on the wall of the cell. This point, 
therefore, is of more ideal than practical importance for the 
prosecution of the investigation. 

The third point relates to the signification which attaches 
to the individual parts of the germ-vesicle. It may be hol- 
low consistently with both views. Although we are not as yet 
acquainted with any hollow nuclei in plants,’ we have never- 
theless found nuclei in cartilages which were hollow, and de- 
cidedly to be regarded as cytoblasts. The question now arises, 
what are Wagner’s spots or spot? If the germ-vesicle be con- 
sidered to be a young cell, one of them may be its nucleus, and 
the rest cell-contents, or nuclei of young cells, which will be 
developed afterwards ; if it be regarded as nucleus, the spots 
may either be nucleoli, or merely its contents. It is a fact in 
favour of the former view, that only one spot is present in 
most instances, the others being usually produced at a later 
period. Wagner has sometimes observed one or more minute 
points in this single spot, and has delineated them from Alcedo 
hispida, Lepus cuniculus, Ovis aries, &c.; I have also sometimes 
met with small points of this kind which gave the spot, in some 
degree, the appearance of a nucleus adhering to the wall of the 
cell, and containing within it these little pomts as its nucleoli. 
Meanwhile, their presence is too inconstant, and they are gene- 
rally too indefinite, to permit of our attributing any importance 
to them in the decision of the present question. The extra- 
ordinary number in which they frequently occur is opposed to 
their being regarded as nucleoli within the germ-vesicle, pre- 
suming it to be a cell-nucleus, for in fishes they sometimes fill 
the entire vesicle, at least, being closely crowded, they cover 


' See Note, p. 33. 


48 THE OVUM AND 


the internal surface of it. Three is the largest number of 
nucleoli which I have observed in other nuclei, and Schleiden 
has in some very rare instances seen four in plants. If, how- 
ever, they are only the contents of the nucleus, and not 
nucleoli, it must be allowed that they differ very much from 
the contents of almost all other nuclei, which are generally 
yellowish, and made up of extremely minute granules. The 
only exception which I have met with was that already men- 
tioned respecting the nucleus of the adipose cells in the cranial 
cavity of a young carp. This last point seems therefore 
rather in favour of the germ-vesicle beg regarded as a 
young cell.! 

When the whole of the above detailed evidence is reflected 
upon in connexion, it will be seen that it is as yet impossible 
to decide the question as to whether the germinal vesicle be 
cell or nucleus, The opinion that the vesicle is to be regarded 
as a cell-nucleus, seems for the present to have the ascendancy, 
inasmuch as the observations upon the first and most important 
point, viz. the prior existence of the germ-vesicle to that of 
the yelk-cell appear to be in favour of that view.” The sub- 


1 Since in vegetable cells the nucleolus is the primary formation, and the nucleus 
a secondary one around it, and as the same has been shown to be most probably the 
case in animal cells, (see page 20, on the production of the nucleus of cartilage- 
cells,) so also in this case the signification to be assigned to Wagner’s spot depends 
upon the history of the development of the germ-vesicle. The observations of 
Wagner, quoted in the Supplement, show, however, that the single germinal spot of 
the ova of insects is first formed, and the germinal vesicle afterwards around it. 
The former must then be considered as nucleolus to the vesicle, which corresponds 
to the nucleus. When several of Wagner’s spots occur, their signification is totally 
different from that of the first one, and they are to be regarded only as secondary 
formations in the interior of the germ-vesicle. In fact, the younger the ova of fishes 
and frogs, the fewer spots are observed in them. 

2 The following is the probable course of formation of the ovum, according to the 
researches now before us; the ovisac (Eisach, ovisac of Barry, internal mem- 
brane of the Graafian vesicle) is first developed. In this (according to analogy 
with Wagner’s observations on the ova of insects) a germinal spot is generated, as 
nucleolus to the ovum. Around that spot the germinal vesicle is formed as nucleus 
to the ovum; and round this again the oyum-cell (Eizelle.) Martin Barry, in- 
deed, (I. c. p. 308,) conjectures that the germ-vesicle is formed previously to the 
ovisac; but my respected friend expresses himself with great caution on the ques- 
tion; and it would in fact be difficult to determine whether a given vcsicle were a 
germinal vesicle, around which no ovisac had as yet formed, or an ovisac within 
which no germ-vesicle had as yet formed. The occurrence also in the lower ani- 


GERMINAL MEMBRANE. 49 


sequent relations of the vesicle seem also to afford evidence in 
its favour. The disc, for instance, is formed around it, and 
this perhaps corresponds to the granulous precipitate which 


mals of several ova in one ovicapsule is difficult of explanation by Barry’s view. 
In the further investigation of this subject, attention must continue to be fixed 
upon the possible, and even probable, existence of a nucleus to the ovicapsule. 
Wagner saw certain follicles in the mole, in which he could not detect a trace of 
any enclosed body. 

Wagner expresses himself in his new work (Lehrbuch der Physiologie, Leipzig, 
1839, p. 34) as being doubtful whether the vesicles met with in his observations 
on the preformation of the germinal vesicle in the ova of insects, were actually 
vesicles or not. The observations of Barry on the ova of mammalia and birds, 
are, however, in favour of the explanation of the ovum of the insect originally 
given by the first-named highly respected investigator, and therefore also of 
that which represents the germ-vesicle as nucleus of the ovum-cell. It is 
true it might be said, that, regarding the germ-vesicle as a cell, a second one, 
the ovum-cell was formed around it; but as opposed to that view, it must 
be remembered that no example of a second cell being formed around the first is 
afforded amongst all the other cells which exhibit a nucleus of the decidedly cha- 
racteristic form. The point in dispute, as to the interpretation to be placed upon 
the germ-vesicle, loses, however, somewhat of its importance if the theory which I 
shall propose (see the conclusion of the treatise) be received, inasmuch as [ shall 
there endeavour to prove the formation of the cell around the nucleus to be merely a 
repetition of the process by which the nucleus is formed around the nucleolus, and 
that the whole process of development of the cell may be reduced to a single or 
many times repeated formation of strata. The germinal-vesicle accordingly is the 
first stratum, or a cell of the first order; the yelk-cell the second stratum, or a cell 
of the second order. As above stated at page 47, a minute point was observed in 
the germinal spot by Wagner, and subsequently by myself also; and my respected 
colleague Vanbeneden lately found germinal spots in the ova of certain polypes 
(Genus Zoanthus), and also in ova of Anodonta, which had not as yet left the ovary, 
that appeared granulous, but at the same time seemed to be hollow, and some of 
which distinetly contained a very small round corpuscle. This observation accords 
most completely with the theory which regards the cells as produced by a stratified 
formation. This small corpuscle, which may be called a secondary nucleolus, would 
here be the primordial formation; the germinal spot would be the first stratum 
around it, that having in this instance become developed into a vesicle, in a manner 
likewise to be explained hereafter by the Cell-Theory; the germinal vesicle would 
be the second, and the yelk-cell the third stratum. The formation of even a fourth 
stratum, the albumen membrane, around the yelk-cell, would involve nothing con- 
tradictory to the theory ; but in such case we certainly could not avoid regarding it 
as a second cell, which had become formed around a previously existing one: for 
the yelk-cell cannot well be considered to be a nucleus. The mode of formation of 
this albumen membrane must, however, in the first instance, be ascertained by in- 
vestigation. 


4 


50 THE OVUM AND 


usually takes place around the nucleus in other cells; and again, 
the germ-vesicle disappears, precisely as the nucleus of other 
cells is generally absorbed. There is then no evidence that the 
fluid of the germinal vesicle exercises a fructifying imfluence ; 
but if it be the cell-nucleus, it disappears, because it has com- 
pleted its office,—the formation of the yelk-cell. The dise, 
which has formed around it, becomes developed into the 
germinal membrane, and it is uncertain whether the remains 
of the germ-vesicle also take part in that formation. 

We shall next proceed to the consideration of the other 
contents which the yelk-cell includes in addition to the germ- 
vesicle, making use of the bird’s egg for the purpose. Setting 
aside some points of distinction of slighter importance, the 
globules, well known as present in the yelk of the hen’s 
egg when laid, may be divided into two principal classes: a, the 
globules of the yelk-cavity ; and 0b, those of the true yelk-sub- 
stance. The former (a) are not only present in the yelk-cavity, 
but occur also in the canal leading from it to the germinal 
membrane, and in the little prominence, called by Pander the 
nucleus of the tread (Kern des Hahnentritts). When many 
of them lie close together, they exhibit a white colour, whilst 
the true yelk-globules in such circumstances appear yellow. 
They may also be distinguished from the latter globules under 
the microscope, (see pl. II, fig. 2.) They are perfectly round 
globules, with quite smooth edges, each enclosing a smaller one, 
which is also perfectly spherical, and looks like an oil-globule, 
being rendered very distinct by its sharp outline. 

The remaining space in the large globules is usually trans- 
parent, and not granulous. But some may be observed which 
have granulous contents, and they then completely resemble 
the true yelk-globules, except that the latter do not gene- 
rally contain any smaller ones with such dark outlmes. Some- 
times also, the globules of the yelk-cavity contain two or more 
such smaller ones. The common yelk-globules (4), that is, 
those of the true yelk-substance, may be distinguished from the 
above-described by the following characteristics : they are upon 
the whole larger, they have all granulous contents, and, for the 
most part, donot enclose any smaller globules. They are very sen- 
sitive to the action of water, which causes them to fall to pieces, 
and then the granules enclosed within them becoming free, give 


GERMINAL MEMBRANE. 51 


a milk-white colour to the fluid. These granules, which are 
of various size, resemble milk-globules, and, as has been fre- 
quently remarked by others, exhibit also like them a brisk 
molecular motion. In consequence of the speedy action of 
water upon these globules, they must be examined in albumen 
or a weak solution of common salt, which preserves them better. 
These fluids also do not impart a white colour to the surface of 
a yelk which is opened in them, as water does. The globule, 
when crushed under the compressorium, tears somewhat sud- 
denly on one side, the other margins remaining smooth, and 
then, without any increase of the pressure, a large quantity of 
the globules contained in it flow slowly forth. This fact indicates 
an external membrane belonging to the globules, but it must be a 
very soft and delicate one.. Baer, who distinguishes four kinds 
of them, believes that he has also sometimes seen such a mem- 
brane in the yelk-globules of immature ovarian eggs. The 
yelk-globules when isolated are round, but, in their natural 
position in the yelk, they flatten against one another into 
angular shapes, in which manner the crystal-like bodies observed 
by Purkinje in the boiled yelk are produced. These bodies 
generally make up the whole of the true yelk-substance of a 
fresh egg, so that, with the exception of the contents of the 
yelk-globules, we do not usually meet with any free granulous 
substance in the yelk. The minutely granulous substance 
which is observed in addition to the yelk-globules, particularly 
after the action of water upon them, appears in most instances, 
and on the external layers of the yelk invariably, to be produced 
solely by the destruction of the yelk-globules. In the vicinity 
of the yelk-cavity of a boiled egg, however, we frequently find 
a coagulated substance composed of granules similar to those 
contained in the yelk-globules, and which appears to be actually 
free yelk substance not enclosed within globules. 

It is necessary to examine the eggs while still contained in 
the ovary, if we wish to become acquainted with the process of 
formation of these two kinds of globules (those of the yelk- 
cavity and yelk-substance), and the mode of production of the 
yelk-cavity and its canal. The younger eggs, having a diameter 
of one or two lines, have a grayish-white colour, but are not 
yellow ; if such an one be cut through the centre, under water, 
it is found to contain a thick, semi-fluid, grayish-white mass, 


52 THE OVUM AND 


part of which flows slowly out. Around this mass lies a more 
consistent, cohering, membrane-like stratum, which lines the 
cavity of the little egg. When a portion of this mass is exa- 
mined under the microscope, a great many round and very trans- 
parent vesicles or cells are observed in it, each of which encloses 
a dark corpuscle resembling an oil-globule. Many such globules 
float about free, and in addition to them there is also a good 
deal of minutely granulous substance present. In order, how- 
ever, to examine this mass in a perfectly natural condition, the 
use of water must be avoided; one of the little eggs, of from 
half a line to a line in diameter, should be placed upon the dry 
object plate, and then pierced, a drop of its contents bemg 
allowed to flow out. This drop will be found to consist entirely 
of very pale cells, most variable in size, each one containing a 
round globule, the size of which is about proportionate to that 
of the cell. This globule or nucleus resembles an oil-globule, 
in consequence of its dark outline, (see pl. II, fig. 3.) Many 
of these cells with their nuclei are so small, that, when lying 
close together, they might be regarded as a merely granulous 
substance; the cells may, however, be recognised with a fa- 
vorable light. Some of the larger ones occasionally contain 
two or three of the globules or nuclei before mentioned. The 
contents of the cells are usually quite transparent, but some 
isolated ones are seen, in which a minutely granulous precipi- 
tate has formed. These cells are enclosed within the egg, in 
a small quantity of transparent fluid. In order to explain’ the 
somewhat variable appearance which the contents of the egg 
assume after contact with water, a small one should be placed 
upon a glass with a drop of that fluid, and some of its contents 
pressed out whilst under the microscope. A quantity of these 
cells will then be seen to burst quite suddenly in the water, 
precisely like soap-bubbles in the air. In consequence of their 
paleness, the fact of the bursting is rendered manifest, in the 
first instance, only by the sudden motion of the nucleus, which, 
together with some minutely granulous substance, remains 
behind. If these cells were solid, although ever so soft, this 
sudden bursting would not be possible. They are therefore 
true cells. I cannot say whether the globule enclosed in them 
is to be regarded as the nucleus. Although it resembles an 
oil-globule, it does not appear to be fat; for if acetic acid be 


GERMINAL MEMBRANE. 53 


applied to a drop of the contents of the egg, it does not appear 
to act materially upon the cells, and the contained corpuscle 
becomes paler and somewhat swollen, which could not well 
take place if it were fat. These cells, then, are the earlier 
stage of development of the subsequent globules of the yelk- 
cavity. The larger ones already resemble them perfectly. 
These globules of the yelk-cavity are therefore likewise cells. 
Their nucleus-globule (Kernkugel) is acted on by acetic acid 
precisely in the same way as it was in the earlier condition. 
It does not lie centrally in the cell, but on the internal surface 
of the wall, as is seen when the cells are caused to roll under 
the microscope. When at rest, however, they are generally so 
placed that the nucleus-globule occupies the most depending 
point (because probably it is the heaviest portion of the cell), and, 
on that account, it then appears to lie in the centre of the cell. 
The yelk in the first instance contains only the yelk-cavity, 
with its cells; the proper yelk-substance with its globules not 
being as yet formed. The colour of these young eggs is there- 
fore also white, like the contents of the yelk-cavity. 

The membrane-like layer which surrounds the above-described 
contents of the egg, may be completely separated from the parts 
which surround it externally with facility, after the egg has 
been divided through the centre. It is not connected with 
them, and appears, to the unaided eye at least, to be pretty 
smooth on its external surface ; it is not possible to trace it 
towards the interior. Its structure is peculiar. Purkinje, who 
discovered it, describes it as consisting of globules, which re- 
semble in form and size, but are more transparent than the 
blood-corpuscles. When spread out upon a plate of glass, and 
examined with the microscope, it is seen to consist of two parts, 
an internal minutely granulous stratum, and an external layer 
of cells. Numerous little granules are observed in the internal 
stratum, which resemble the nuclei of the above-described cells 
of the yelk-cavity in their earliest stage, and I conjecture that 
the cells of the yelk-cavity are formed from this stratum, so 
that in fact it still pertaims to the yelk-cavity. The external 
layer consists of small round granulous cells, each of which 
contains a nucleus, which again in many instances encloses 
one or two nucleoli. ‘Two or three such layers of cells lie one 
above another. ‘These layers of cells are surrounded externally 


54 THE OVUM AND 


by a very transparent, perfectly structureless membrane, which 
represents a closed cell-membrane, having as little connexion 
with the ovary as with the layers of cells, and which is deno- 
minated vitelline membrane. It is as readily separated from 
the ovary as from the layer of cells, the latter, therefore, cannot 
be merely its epithelium. 

If we now proceed to examine larger eggs from the ovary, 
such, for instance, as have attained a diameter of half an inch 
or more, and are already yellow-coloured, on their being divided 
across the centre under water, a white substance, the yelk- 
cavity, will be found in their interior. This cavity contaims 
those cells, now in a higher stage of development, which in the 
first instance alone formed the contents of the egg. Around 
these a stratum of yellow substance, the proper yelk-substance, 
appears, and round this again lies the layer of cells. Globules 
may be recognised in the proper yelk-substance with the aid of 
the microscope, as in the same substance of the mature yelk. 
These globules, then, have been formed between the yelk-cavity 
and the layer of cells. The question, however, arises how 
this has been effected? The following may be supposed to be 
the mode of their production :—the innermost portion of the 
yelk, the yelk-cavity, is the part which is first formed, the 
innermost yelk-globules are therefore also the oldest, and the 
formation of the new yelk-globules takes place externally upon 
the internal surface of the layer of cells. Ifa small portion 
of the layer of cells be so placed under the microscope that the 
inner surface becomes turned towards the eye, and a spot be 
sought for at which a thin layer of yelk-substance is attached 
to it, it will be seen that the yelk-globules do actually become 
smaller in the proximity of the layer of cells, whilst in other re- 
spects they retain their general appearance. The smallestof them, 
which le immediately upon the inner surface of the layer of 
cells, are even smaller than the cells of the layer itself. It is 
therefore extremely probable, that the formation of new yelk- 
globules takes place on the inner surface of the layer of cells, 
and that the globules then expand to their normal size some- 
what quickly, for the stratum of small ones is but thin. Mean- 
while new ones continue to form externally, until the yelk has 
reached its normal size. The formation of the canal leading 
from the yelk-cavity to the germinal vesicle may also be ex- 


GERMINAL MEMBRANE. 55 


plained in the same manner ; for instance, no formation of yelk- 
globules can go on at that point at which the germ-vesicle and 
the stratum for the germinal membrane are in connexion with 
the layer of cells, but at that spot there must be a gap in each 
stratum of yelk-globules, which by the increasing thickness of 
the yelk-substance becomes a canal, necessarily conducting from 
the yelk-cavity towards the germinal membrane, and imto which 
cells from the yelk-cavity become crowded. Now are these 
globules of the proper yelk-substance cells? I cannot prove 
decisively that they are so; the following arguments, however, 
render it probable: 1st, because Baer believes that he observed 
an external membrane in some of them; 2dly, because, when 
ruptured at a particular spot by the compressorium, they at 
once pour out a large portion of their contents without the 
pressure being increased ; 3dly, because, notwithstanding that 
they le close together in the yelk and flatten against one 
another, they do not run together; 4thly, because they so 
closely resemble some of the cells of the yelk-cavity which are 
furnished with granulous contents; 5thly, because they, like 
cells, appear to have an independent growth. ‘These reasons 
are sufficiently strong to render it probable that the yelk- 
globules have a cellular structure, though they cannot be received 
as decisive of the point. However, inasmuch as they all form 
the contents of a larger cell, it is not absolutely necessary for 
our purpose that they should be distinctly proved to be cells. 
Both the indubitable cells of the yelk-cavity, and those proble- 
matical ones of the proper yelk-substance, have an independent 
growth in a fluid, and within another cell. They are cells 
within cells. For although the formation of new cells takes 
place only at the outside, yet they are still separated from the 
organized substance, not only by the cell-membrane of the 
entire ovum, but also by the layer of cells which is situated 
immediately beneath it. We here, then, meet with an 
instance of just such a formation and independent growth 
of cells within a fluid as was expressed by the fundamental 
phenomenon previously laid down. It is a point open to in- 
vestigation, whether the cleaving of the yelk described by 
Baer, Rusconi, and others, in the development of the lower 
animals, the ova of frogs for example, may not also depend 
upon a process of cell-formation, two cells being developed 


56 THE OVUM AND 


within the yelk in the first instance, and in each of these again 
two mew ones, and so on. 

We next proceed to consider the changes undergone by the 
external layer of cells furnished with nuclei. In eggs which 
have a diameter of a line, this entire membrane, if it may be so 
called, appears to be made up merely of cells. In such as have 
reached a higher stage of development, such as have a diameter of 
upwards of half an inch, for instance, it consists of two strata, 
the external of which is granulous, and no longer exhibits cells ; 
the internal, however, is composed of cells, which are flat, 
hexagonal, but also granulous, and bear the relation of a cover- 
ing of epithelium to the outer one. The external stratum 
passes away over the germinal vesicle and the foundation of the 
germinal membrane, so that these structures may easily be re- 
moved from its inner surface without injury toit. The internal 
cellular stratum, on the contrary, is interrupted at the spot where 
the germinal vesicle lies. I have not traced the mode of formation 
of this external granulous stratum through all its details ; I sup- 
pose it to be produced bya blending of the outer cells, which com- 
posed the original membrane when it was made up entirely of 
cells. As the period approaches at which the egg leaves the ovary, 
the epithelium-like stratum of cells gradually disappears, and the 
granulous membrane alone remains. It does not exhibit any 
disposition to unite with the structureless external membrane 
of the egg, even in eggs which are almost sufficiently mature for 
extrusion. If such an egg be cut open under water, and the 
investment derived from the ovary be drawn off, this granulous 
membrane frequently remains lying upon the yelk, whilst the 
structureless membrane follows the above-mentioned investment, 
and may readily be proved to be connected with it, when they 
are folded so that the inner surface forms a sharp edge. By 
the aid of the compressorium this structureless membrane may 
then be seen, projecting out from the border of the preparation. 
It often separates in large pieces during this manipulation, so 
that it has likewise no connexion with the parts pertaining to 
the ovary. If the signification of vitelline membrane is to be 
assigned to this structure, a blending between it and the granu- 
lous stratum must take place in the oviduct, in order to form 
the subsequent vitelline membrane of the extruded egg. 

We now pass on to that portion of the egg from which the 


GERMINAL MEMBRANE, 37 


embryo is first formed, the germinal membrane. It represents, 
as is known, a round, white, little disc, somewhat above a line 
in breadth, which lies between the vitelline membrane and the 
yelk-substance. This little disc, in a fresh-laid hen’s egg, con- 
sists of globules, which are of unequal size in different parts of 
the germinal membrane. When examined with the microscope, 
they appear much darker than the yelk-globules, (see plate IT, 
fig. 4.) They le in close contact, so that they flatten against 
one another to an hexagonal form. The boundaries of the dis- 
tinct globules may be clearly distinguished, even when in con- 
nexion. They may also be readily isolated from one another, 
and are then round. They contain many smaller round gra- 
nules of various size, with very dark outlines, which float about 
singly when the globules are burst by pressure. Although these 
granules, in most instances, completely fill the globules, yet some 
globules may be observed where that is not the case, and where 
a portion of the globule is transparent, and free from granules, 
(a 6, of the above figure.) I thought that I distinctly saw a double 
external outline on one of these globules (a), which would be 
evidence of the presence of a cell-membrane. Jn most in- 
stances, however, this is not distinct, and my principal reason 
for concluding that they are cells, is, that it is so extremely 
probable that they are developed to form the indubitable cells 
of the incubated germinal membrane. I have not, however, 
fully investigated this process, and only communicate my ob- 
servations on the point, incomplete as they are. If the unin- 
cubated germinal membrane be folded in such a manner that 
its external surface form a sharp margin, that surface is found 
to be tolerably even, dark, and composed immediately of the 
globules of the germinal membrane already described ; the sur- 
face of the germinal membrane of an egg which has been ex- 
posed to brooding heat for four hours, presents a precisely 
similar appearance. The same membrane, when examined also 
upon its general surface, differs but very slightly in appearance 
from one which has not undergone incubation. The globules 
of which it consists merely appear to have more minutely 
granulous contents. But if a germinal membrane after eight’ 


' It is quite as impossible to define with any certainty a fixed time for a precise 
stage of development of the elementary cells of the germinal membrane, as it is to 
connect the formation of the area pellucida, the embryo, and its separate parts, with 


58 THE OVUM AND 


hours’ incubation be folded in the same manner, its margin at 
many points is found to be no longer dark and even, but to be 
composed of extremely pale transparent cells. These cells pre- 
sent every variety of size, some being as large and even larger 
than the primitive globules of the germinal membrane. They 
either project forward in the form of half-spheres, or the greater 
portion of their spherical surface juts out in some instances, and 
they may be completely separated by pressure. They contain a 
pellucid fluid, but no nucleus. ‘The following fact shows them 
to be cells; some of them contain very minute, isolated, black 
granules, which resemble the molecules described by Brown, 
and exhibit molecular motion within the cell. This fact proves 
that the contents of the cell must be fluid. A fluid which is 
miscible with water cannot, however, preserve any definite form, 
unless it be encompassed by a membrane. Such a structure must, 
therefore, exist in this instance. It is not altogether easy to con- 
vince one’s self that these granules, exhibiting molecular motion, 
do actually le within the cells ; but it may be concluded from the 
fact, that they do not flow away when the surrounding fluid is 
allowed to escape, and that they are not moved beyond the 
limits of the cell, but only to its walland back again. Beneath 
this stratum of cells lie the globules of the unincubated germinal 
membrane, which, however, appear to have become still more 
clear and minutely granulous than those of the membrane ex- 
amined after four hours’ incubation. In addition to these, se- 
parate cell-nuclei may be observed, such as occur in the cells 
of the serous layer at a subsequent period, and may be seen in 
plate I], fig. 6. Still more internally than this layer, we meet 
with perfectly dark globules. The serous and mucous layers of 
the germinal membrane are perfectly formed in the egg after 
sixteen hours’ incubation. If the membrane at that period be 
folded so that its external surface may be seen, it will be found 


any degree of certainty to any precise hour of incubation. The periods cited should 
therefore only be taken as being near about the true determinations of the time. 
The cells in the germinal membrane, before incubation even, do not appear to be al- 
ways at the same stage of development; thus, plate IJ, fig. 4, ec, and fig. 4, a, 0, re- 
presents cells from two different membranes. A great portion of the germinal mem- 
brane from which ¢ was taken consisted of such cells as that delineated, and I thought 
I perceived molecular motion in the granules contained in some of them, which, if 
correct, would clearly prove them to be cells. 


GERMINAL MEMBRANE. 59 


to be composed of cells, which project forwards in the form of 
half-spheres, (plate II, fig. 5). A nucleus of the characteristic 
form may be recognised in some of them. It lies upon the in- 
ternal surface of the cell-wall, is round, and contains one or two 
nucleoli. In most instances, however, no nucleus can be seen, 
either because none is present, or because it lies upon the 
posterior side of the cell, in which position it cannot be per- 
ceived, in consequence of the dark substance lying beneath it. 
The cells also contain a transparent fluid, and some minute gra- 
nules with molecular motion, which is evidence sufficient for 
the existence of a peculiar cell-membrane. If, after the ger- 
minal membrane has lain for a time in water, the mucous layer be 
washed off, the general surface of these cells may be observed. 
They are then seen to lie close together, and to flatten against 
one another to hexagonal forms, (see plate II, fig. 6). They 
contain a beautiful nucleus, which encloses one or two nu- 
cleoli. They also present many minute granules, which ex- 
hibit molecular motion. The cells may also be observed in the 
recent germinal membrane, especially on its margin, at which 
part it is more transparent, and there they project forward in 
the form of large segments of a sphere. These cells then re- 
present the serous layer of the germinal membrane,—which, 
therefore, consists of round cells (their polyedrical form being 
referrible solely to their lying so closely together), furnished 
on the inner surface of their wall with the characteristic nucleus, 
and containing a clear fluid, and some isolated smaller granules. 
They might be conceived to be a mere covering of epithe- 
lium to the serous layer. But if the serous layer be separated 
after the blood has formed, for example, in an egg which has 
undergone forty-eight hours’ incubation, the vascular layer re- 
mains lying immediately upon this stratum of cells. Valentin 
has already recognised these cell-nuclei, for he says, that 
each of these layers of the germinal membrane consists of a 
transparent vitreous jelly, but that they are to be distinguished 
by the corpuscles which they contain. (Entwicklungsgeschichte, 
page 287.) These corpuscles are the cell-nuclei, the trans- 
parent substance in which they le is composed of the cells, and 
is gelatinous only in appearance. The cells have only a mi- 
nimum of intercellular substance between them. 

When, in the next place, we proceed to examine the mucous 


60 THE OVUM AND 


layer of the germinal membrane of an egg after sixteen hours’ 
incubation, we find it to be composed of globules, which vary 
greatly both in size and appearance, (see plate II, fig. 7.) The 
large globules, which form the greater proportion, may be proved 
to be cells, and Baer has already named them vesicles. The 
molecular motion, which is frequently visible in isolated globules 
within them, although much slighter in these instances than 
in the cells of the serous layer, affords sufficient evidence of 
their cellular character. They contain a transparent fluid and 
granules of various kinds. One particular globule, having very 
dark outlines, resembling those remarked in the cells of the 
yelk-cavity, may be observed in almost every cell. Several of 
the globules, and of all gradations of size, are frequently seen 
in a cell. In addition to the above, a minutely granulated 
substance is present in many of them. These cells lie some- 
what loosely together in a structureless, tenacious, intercellular 
substance, which is their cytoblastema, so that at this stage 
they are but slightly flattened against one another. This in- 
tercellular substance contains, in addition, perfectly dark glo- 
bules and smaller granules, but I do not know what relation 
they bear to the cells. A portion of them may, perhaps, be 
nuclei of new cells. Yet I could not decide whether the one 
dark globule, which is generally so very prominent in the cells 
of the mucous layer, had actually the signification of a cell- 
nucleus. It differs in form from the usual cell-nucleus very 
materially. During the progressive development of the ger- 
minal membrane, the quantity of intercellular substance, and 
of those globules the cellular nature of which is not demon- 
strable, diminishes very much, so that at a subsequent period 
the cells lie close together, and present the appearance of ve- 
getable cellular tissue. The description here given applies 
only to the mucous layer on the outside of the area pellucida. 
Within that the cells have quite a different appearance. They 
are very much smaller, of pretty equal size, very transparent, 
and contain no coarse granules, but only very small globules. 
They do not appear to have any nucleus, and this fact dis- 
tinguishes them from the cells of the serous layer, which pos- 
sess a nucleus even within the area pellucida. 

The first rudiments of the embryo appear to be formed from 
the cells of the serous and mucous layers of the germinal mem- 


GERMINAL MEMBRANE. 61 


brane, that is, from such cells as are met with in the area 
pellucida, so that the embryo is composed, partly of small 
cells without nuclei, and partly of cells furnished with the cha- 
racteristic nucleus. It presents, however, besides them, an 
extraordinary quantity of simple cell-nuclei with nucleoli, around 
which no cells have as yet formed. 

I have made but few researches with respect to the structure 
of the vascular layer, and from them, I could not (with the 
exception of the vessels themselves and the blood) detect any 
such essential difference between it and the mucous layer, as 
was exhibited between the latter and the serous layer. As, how- 
ever, the formation of the vessels themselves, although it ap- 
pears to depend upon a production of cells, is not a process pe- 
culiar to the germinal membrane, we shall defer it, to be re- 
sumed at a subsequent stage of our investigation. 

I have not ascertained the relation which these cells of the 
layers of the gerrainal membrane have to the primitive globules 
of the membrane before incubation, or within eight hours after 
that process has commenced ; but inasmuch as it is probable 
that at least one of those kinds of cells owes its origin to the 
development of the primitive globules, we may be permitted to 
suppose that those globules are likewise cells. 

For the purpose of giving, in outline, a connected view of the 
changes which the egg undergoes, from its first formation up 
to the period at which the actual development of the embryo 
commences,—in so far as the foregoing, more or less complete, 
observations enable us to form a provisional conception of the 
process of development,—we will proceed on the understanding, 
that the germ-vesicle is the nucleus of the yelk-cell ; at the same 
time, however, we expressly refer the reader to the more de- 
tailed statement above furnished for the certainty both of this 
and of every other separate point which occurs in the following 
exposition. Itis probable that the germ-vesicle is the first struc- 
ture, and that the yelk-cell forms around it as its cell-nucleus. 
Both advance in growth, the latter, however, much more rapidly 
than the former. A precipitate, the commencement of the ger- 
minal membrane, next forms around the germ-vyesicle. Young 
cells are simultaneously formed in the remaining space of the 
yelk-cell, these are the cells of the subsequent yelk-cavity. Then 
cells of another kind originate beneath the vitelline membrane, 


62 THE OVUM AND 


which are the subsequent cells of the proper yelk-substance. 
They are formed round about the neighbourhood of the vitelline 
membrane, with the exception of that spot where the germ- 
vesicle and the rudiments of the germinal membrane lie. These 
cells expand very rapidly, while at the same time a new layer 
is formed on the outside of them, and so on successively. In 
this manner they surround the white cells of the yelk-cavity 
with a layer of yellow cells, which is constantly increasing in 
thickness ; as, however, a vacant space remains at the spot where 
the germinal vesicle and germinal membrane are situated, by 
the increasing thickness of the yelk-substance, the space be- 
comes converted into a canal. The development of the vitelline 
membrane proceeds continuously with these changes, in pro- 
portion as the increasing contents require. When the yelk- 
cell has attained its due size and the egg leaves the ovary, the 
germ-vesicle, like most other cell-nuclei, disappears, and the now 
more fully developed germinal membrane remains. It is made 
up of globules, probably cells, having coarsely-granulated con- 
tents. It grows during the process of incubation by the con- 
tinual development of new cells. After sixteen hours’ incuba- 
tion, a distinction may be observed in the cells composing the 
membrane. The more external ones form a layer, in which the 
cells exhibit a nucleus of the characteristic form, and contain 
a quantity of transparent fluid and minute isolated granules. 
These cells are therefore clear, and firmly united together, and 
have only a minimum of intercellular substance between them ; 
they represent the serous layer of the germinal membrane. The 
under stratum of the germinal membrane or mucous layer con- 
tains cells of another kind; they have no nucleus of the cha- 
racteristic form, but contain one or more dark globules, and 
frequently also some minutely granulous substance. These cells 
lie loosely together in a larger quantity of imtercellular sub- 
stance, which contains smaller granules of different kinds, in 
addition. When this division of the membrane into the two 
layers is completed, and its superficies has become considerably 
extended, and after a transparent spot, the area pellucida, has 
formed in its centre—(the cells of the mucous layer in this 
area being much smaller, but of pretty equal size, as com- 
pared with one another, and having transparent contents with 
very minute isolated granules),—the embryo is developed, 


GERMINAL MEMBRANE. 63 


as a portion of the germinal membrane separating from the 
whole by a constriction. Both layers contribute to its forma- 
tion, and it therefore consists of small transparent cells, some 
of which (probably those pertaining to the mucous layer) con- 
tain no nucleus, whilst others (those derived from the serous 
layer) exhibit the characteristic cell-nucleus with its nucleoli. 
In addition to these cells it contains a great many nuclei, 
around which no cells have as yet formed. Between the 
two layers of the germinal membrane other cells arise, which 
may be regarded as representing a third layer, the vascular, 
although they do not really form a connected independent 
layer; of these we shall treat hereafter. These three layers 
then, and pre-eminently the first two, form the mediate basis 
of all the subsequent tissues. 

The yelk is not a lifeless aliment for the embryo,—as it is 
when taken as food by the adult, to whose organism it is dead 
and must be chemically dissolved,—but the cells of the yelk 
take part in the vitality called forth by incubation. They 
effect an alteration in their contents, whereby the albumen 
which they contain loses its property of coagulating, and the 
granules become dissolved, in the same manner in which the 
granules of starch dissolve in the cells of the vegetable embryo. 
In short, the yelk bears the same relation to the embryo as 
regards its nutritive property, that the albumen bears to the 
vegetable embryo. 

In accordance with the analogy between the cells we are 
treating of and those of vegetables, all the changes in the egg, 
the growth of the germinal membrane, and even the first forma- 
tion of the embryo, proceed entirely without vessels. 


64 PERMANENT TISSUES OF 


SECOND DIVISION. 
Permanent Tissues of the Animal Body. 


The foregoing investigation having taught us that the entire 
ovum, from its first origin up to that period at which, by the 
formation of the serous and mucous layers of the germinal mem- 
brane, the foundation of all the subsequent tissues is laid, exhibits 
simply a continual formation and more extended development of 
cells, and having found the primordial substance of the tissues 
itself to be Soripehed of cells, we are now required to prove, that 
the tissues do not only originate from cells in this general man- 
ner, but that the special basis of each individual tissue is a matter 
composed of cells, and that all tissues either consist entirely of or 
are formed from cells which pass through a variety of transforma- 
tions. These modifications, which some of the cells undergo in the 
progress of their development to the subsequent tissues, are 
very important, since thereby the cells not infrequently cease 
to exist as separate independent structures. We have al- 
ready (in the Introduction) seen such changes in plants, for 
example, in. the coalescence of the cell-walls observed by 
Schleiden in the bark of the Cacti, and the blending of several 
cells to form a tube in the spiral and lactiferous vessels. This 
takes place to a much greater extent in animals, and, in general, 
the higher the importance of a tissue is, the more do the cells 
lose their individuality. We shall not, however, enumerate 
these modifications here; we shall become acquainted with them 
as the result of investigation of the separate tissues, and, at 
the conclusion of the work, we shall combine them into a con- 
nected representation of Cell-life. It is necessary, however, to 
mention the most important of them at least preliminarily in 
this place, in order to make a classification of the tissues. 

Since all organic structure is primarily formed from cells, 
the most scientific classification of general anatomy would 
manifestly be one founded upon the more or less high de- 
gree of development at which the cells must arrive, in order 
to form a tissue. The complete retention, or relinquishment, 


THE ANIMAL BODY. 65 


to a greater or less extent, of their individuality by the cells, 
should serve as the scale for their degree of development. We 
give the name of independent cells to those in which the wall 
remains distinguishable from the neighbouring structures 
throughout the whole progress of its expansion. We apply the 
term coalesced cells to those in which the wall blends, either 
partially or entirely, with the neighbouring cells, or intercellular 
substance, so as to form an homogeneous substance. The cell- 
cavities, in such instances, are separated from one another only 
by a single wall, as we have already observed in cartilage. This 
is the first degree of coalescence ; the cacti present an example 
of it in vegetables. The second is that in which the walls of 
several cells lying lengthwise together, coalesce with one another 
at their points of contact, and the partition walls of the cell- 
cavities become absorbed. In this way not only the walls but 
the cavities of the cells also become united, as in the spiral and 
lactiferous vessels in plants. 

Upon these more or less important modifications of the 
Cell-life the following classification of the tissues is based : 
Ist. Isolated, independent cells, which either exist in fluids, or 
merely lie unconnected and moveable, beside each other. 2d. 
Independent cells applied firmly together, so as to form a 
coherent tissue. 3d. Tissues, in which the cell-walls (but not 
the cell-cavities) have coalesced together, or with the intercel- 
lular substance. Lastly, tissues in which both the walls and 
cavities of many cells blend together. In addition to these, 
however, there is yet another very natural section of the 
tissues, namely, the fibre-cells, in which mdependent cells are 
extended out on one or more sides into bundles of fibres. The 
naturalness of this group will form my excuse for sacrificing 
logical classification to it, and inserting it as the fourth class 
(4th), consequently, that last mentioned, consisting of tissues, 
in which the cell-walls and cell-cavities coalesce, becomes the 
fifth (5th). 

All tissues of the animal body may be comprised under these 
five classes; the classification, however, gives rise to some 
difficulties. For instance, the fibres of cellular tissue and fat 
must be placed in very different classes, so also the enamel of 
the teeth and the proper dental substance. <A second diffi- 
culty arises from the fact, that transitions take place, the 


5 


66 PERMANENT TISSUES. 


isolated cells, for example, passing over into those with 
blended walls; and again, a tissue which usually consists of 
isolated cells, occasionally exhibits in different situations coa- 
lesced cells. Such difficulties, however, present themselves in 
all classifications of natural objects. Nature is very unwilling 
to accommodate herself to our schemes. The object of her aim 
is quite opposed to that of our intellect. She accords and ac- 
commodatesall contrarieties by gentle transitions : the intellect 
disjoins, and seeks everywhere for strongly-marked contrasts. 
If, however, regard be had to the most important structure 
only in each individual tissue,—for example, in the nervous 
system, to the nervous fibres and not to the ganghon-globules, 
in cellular tissue, to its fibres and not to the fat, and so 
forth,—and further, if we regard only that which is the general 
rule as to these structures, all tissues may then be readily 
brought under these five classes. With the desire of making 
this work as complete as possible, I have apphed this arrange- 
ment to all the tissues in the way which has appeared to be 
most probably correct, according to the investigations I have 
hitherto made. Those researches are, however, far from com- 
plete, and continued observations may perhaps render it 
necessary, at some future time, to assign a different position 
to some of the tissues. This may serve as a preliminary 


sketch : 


Class I. Isolated, independent cells. To this class the cells 
in fluids pre-eminently belong ; Lymph-globules, Blood- 
corpuscles, Mucus- and Pus-corpuscles, &c. 


Class II. Independent cells united into continuous tissues. 
Such as the Horny tissues and the Crystalline lens. 


Class III. Cells, in which only the cell-walls have coalesced: 
Cartilage, Bone, and the substantia propria (ivory) of the 
Teeth. 


Class IV. Fibre-cells: Cellular (areolar), Fibrous, and 
Elastic tissue. 


Class V. Cells, in which both the cell-walls and cell- 
cavities have coalesced: Muscle, Nerve, Capillary 
vessels. 


Se Se eee 


» eee 


a 


ISOLATED INDEPENDENT CELLS. 67 


CLASS I. 
Isolated, independent Cells. 


By the above term we understand cells which either float 
free in fluids, or, at least, are moveable, though lying in close 
contact. Such cells, therefore, possess the highest degree of 
individuality. This class includes the cells of lymph, blood, 
and the various secretions. The ovum might be placed at the 
head of this class in a system of general anatomy; but the 
plan of the present work required that it should be discussed 
previously. 


1. Lymph-corpuscles. According to Vogel’s description 
(Physiologisch-pathologische Untersuchungen tiber Hiter, &c. 
Erlangen, 1838), the lymph-corpuscles appear to be cells, 
although he does not express the fact in words. For ex- 
ample, after the corpuscles have been exposed to the action of 
acetic acid, a nucleus is brought mto view, the production of 
which I do not suppose to be referrible to a separation into 
envelope and nucleus, but believe it to have been previously 
formed, and rendered visible solely in consequence of the 
greater degree of transparency acquired by the envelope, i. e. 
the cell-membrane, and its contents, from the action of the 
acid upon them. One of the nuclei, amongst the lymph-cor- 
puscles, delineated in the above-mentioned work (fig. 4 6) 
appears to contain a nucleolus in its centre. I have not 
made any researches myself upon this subject. The mode of 
production of the lymph-corpuscles has not as yet undergone 
investigation. They are probably formed in the lymph- 
plasma, which serves as their cytoblastema, in accordance with 
the general law before laid down. We cannot as yet decide 
the question whether the nuclei are present before the cells, 
and whether the latter are first formed around them; perhaps 
the small granules which Vogel delineates from lymph are 
young nuclei. 


2. Blood-corpuscles. C.H. Schultz was the first who proved 


68 BLOOD-CORPUSCLES. 


the blood-corpuscles to be vesicles.' He relied especially upon 
the manner in which they were acted on by water, whereby 
they lose their colouring matter, swell, and become round, and 
under which circumstances he frequently saw the nucleus roll 
about within the round and very transparent vesicle. The last 
fact would of itself be sufficiently conclusive. JI have not as 
yet observed this fact; on the contrary, in most instances the 
nucleus decidedly adheres to the internal surface of the wall 
of the vesicle, eccentrical as in all cells, though it may pro- 
bably also sometimes become detached. The fact, however, of 
the blood-corpuscles becoming swollen and round, renders their 
cellular nature highly probable. If the envelope (hiille) of the 
blood-corpuscle were not a flattened vesicle, it might indeed 
lose its colour and swell in water, but it would retain its flat 
form, like a sponge when filling with fluid. The circumstance 
of the nucleus remaining on the wall during the swelling of 
the blood-corpuscle in water is no accidental appearance ; for 
even in the round blood-corpuscles of a chick, forty-eight 
hours after the commencement of incubation, when they were 
not as yet flattened, I found that the nuclei, which were also 
circular, were not placed in the centre, but lay eccentrical upon 
the internal surface of the wall. The cellular nature of the 
blood-corpuscle, and the signification of its separate parts 
scarcely appear to admit of doubt when regarded in connexion 
with the whole of this mvestigation. It is a flattened cell fur- 
nished with a cell-nucleus, which is fixed to a spot on the in- 
ternal surface of the cell-membrane. The size of the cell as 
compared with the nucleus is not the same in all corpuscles ; 
that of the nucleus is much more constant. The nucleus of 
some blood-corpuscles of frogs which had swollen in water, also 
appeared to me in some instances to be hollow. It also loses 
its flatness in water, but retains its oval figure. I have 

1 [This is clearly an oversight as Hewson not only demonstrated their vesicular 
nature, and called them vesicles, but accurately described their becoming “ changed 
from a flat to a spherical shape,” on the addition of water to the blood, and the falling 
of the nucleus “ from side to side in the hollow vesicle, like a pea in a bladder.” See 
‘Philosophical Transactions,’ 1773, vol. lxiii, Part Il; or, ‘Experimental Inquiries,’ 
Part III, being ‘a Description of the Red Particles of the Blood,’ &c., &e. (published 
after his death), edited by Magnus Falconar, London, 1777; also the very valuable 


republication of Hewson’s Works by the Sydenham Society, edited by George Gulliver, 
Esq., where the reader is particularly referred to pp. 220, 221.—Trans. ] 


a ele i i i it th 


—— 2 


_ 


ot (22 epee 20) eo 


PVP Gar; ot ce eae es 


BLOOD-CORPUSCLES. 69 


never distinctly observed nucleoli in it; occasionally only 
I thought I perceived something of the kind, for instance, in 
the blood-corpuscles of a salamander ; it was not, however, suf- 
ciently evident to permit of my asserting their presence. Cell- 
contents must certainly exist; for if the cell-walls lay imme- 
diately upon one another, the corpuscle must be as much 
thinner on the margins beside the nucleus as the thickness of 
the nucleus amounts to. If it be assumed that the cell-mem- 
brane alongside the nucleus may be so much thicker as thereby 
to produce the almost level side surfaces, the cell-membrane 
must m such case have a thickness equal to the half of that 
of the corpuscle; but it would then be sufficiently thick to 
allow of a double outline being distinguished when it was 
swollen by water ; observation, however, does not detect any 
such appearance. The red colouring matter forms the cell- 
contents. It is difficult to decide whether the cell-membrane 
and nucleus are also coloured, but it is in some degree pro- 
bable that they are so, since otherwise the centre of the 
corpuscle where the nucleus lies must appear white, whilst it 
in fact exhibits a paler red colour. The colouring matter of 
the blood-cells is not contained in granules, as it is in most 
kinds of pigment, but in a state of solution. If the lymph- 
corpuscles be cells, their transformation into the blood-corpus- 
cles may at least be conjectured as taking place by their 
becoming flattened and absorbing colouring matter. Those 
blood-corpuscles in which the envelope (hiille) is smaller in 
proportion to the nucleus, a fact often observed in the frog, 
are probably younger cells. I have made no observations 
upon the formation of the blood-corpuscles in the germinal 
membrane. According to C. H. Schultz (System der Cir- 
kulation, p. 33), the blood-corpuscles in the chick are formed 
round the yelk-globules. (?) The latter are first present, and 
form the nucleus of the blood-corpuscles ; they become sur- 
rounded with a delicate membrane. The vesicle then dilates, 
and at length becomes flattened. This description accords 
excellently with the fundamental laws previously developed, 
and shows that as early as 1836 Schultz had discovered the 
pre-existence of the nucleus of the blood-corpuscle, the for- 
mation of the blood-vesicle around it, and the gradual expansion 
of that vesicle. 


70 MUCUS-CORPUSCLES. 


3. Mucus-corpuscles. The mucus corpuscles have already 
been described as cells, in consequence of their resemblance to 
the ceils of epithelium. They are round globules, enclosing a 
nucleus, which is eccentrical. We already know this to be 
the elementary form of most animal and vegetable cells, and 
the presence and characteristic position of the nucleus, there- 
fore, warrant us in concluding that in this imstance also the 
globule is a cell, although an especial cell-membrane cannot 
be distinguished. Giiterbock discovered that the nucleus of 
the mucus-corpuscle has the peculiar property of splitting into 
two or three smaller corpuscles when acted upon by acetic 
acid, and that the enclosing or cell-membrane is gradually 
dissolved in the same acid. Vogel, indeed, attributes this pro- 
perty to such mucus-corpuscles alone as have been secreted 
by a morbid action, and to pus-corpuscles. But I have been 
informed by Henle that the true mucus-corpuscles (of which, 
according to him, only a very small quantity exist in healthy 
mucus,) exhibit the same peculiarity, and that those which are 
not affected by the acid are true epithelial cells. As I have 
never observed any other cell-nuclei to be similarly acted on 
by acetic acid, the fact marks the distinction between mucus 
and pus-corpuscles and all other cells, and, according to Henle, 
even the youngest epithelial cells do not possess this property, 
so that the mucus-corpuscles differ distinctly from them. It 
appears to be a characteristic of all cell-nuclei that they not 
only are insoluble, but do not even become transparent in 
dilute acetic acid. These, therefore, are peculiar cells, which 
are formed in the fluid of mucus as their cytoblastema, in the 
same manner as the yelk-cells in the fluid of the yelk-ball. 
They become more abundant, when the cytoblastema obtains 
a greater degree of “ plasticity,” as the result of irritation of 
the mucous membrane; and as on the other hand the secretions 
in the normal condition possess but a very small amount of 
plastic force, and some—the urine and bile, for instance— 
have not any; we accordingly find in them but a very few 
cells, or indeed none at all, save some cast-off epithelium. I 
have not investigated the question whether the nucleus exist 
before the cell in the mucus-corpuscles, or upon what the 
division of these nuclei by means of acetic acid depends. 


PUS-CORPUSCLES. 7\ 


4. Pus-corpuscles. Weare entitled to consider the pus-cor- 
puscles as cells, by the same arguments which we applied to 
those of mucus. Vogel, indeed, regards them as identical 
with those mucus-corpuscles which, according to his view, are 
morbidly secreted, but which Henle believes to be normal. 
They are similarly affected by acetic acid, and cannot therefore 
be young epithelial cells, in which, according to Henle, the 
splitting of the nucleus does not take place under similar cir- 
cumstances ; indeed, that property appears to be confined en- 
tirely to the nuclei of the mucus and pus-corpuscles. Vogel 
states that the nuclei of pus-corpuscles are concave. The pus- 
corpuscles are thus peculiar cells which are formed in the 
serum of pus,—i. e. in cytoblastema, exuded during inflamma- 
tion, in increased quantity, and of anomalous composition,— 
precisely in the same manner that mucus-corpuscles originate 
in mucus, and, indeed, as all cells form in their cytoblastema, 
in accordance with the fundamental law already laid down. 
According to the observations of H. Wood, they appear to be 
earliest formed upon the’ surface of the granulations, and for 
the reason that their cytoblastema, the pus-serum, is constantly 
exuding freshest at that part, and therefore possesses in that 
situation the greatest amount of plastic force, as we have 
already observed in reference to the formation of new yelk- 
cells on the outside and in the neighbourhood of the vitelline 
membrane. It is, however, probable that the pus-cells pursue 
an independent growth for a period, as we have seen to be the 
case with respect to those yelk-cells which were far removed 
from the vitelline membrane. It is also most likely that the 
nuclei of the pus-cells are their first formed part, but I have no 
investigations on the subject. The more healthy the pus, the 
greater is its plastic force, and the greater the number of cells 
which are formed in it, so that in healthy pus the quantity of 
serum is very small in comparison with the number of cells. 

I cannot state whether the oil-globules which are present in 
certain secretions, such as milk and chyle, are contained in 
cells or not. I have not been able to detect anything indi- 
cating that they are so in milk; and, according to the theory 
of the secretions, which will be communicated at a subsequent 
stage of the work, there does not appear to be any necessity 
why they should be so. 


“I 
bo 


PUS-CORPUSCLES. 


The low grade of development held by the class of cells now 
under consideration, in which those elementary formations re- 
tain their greatest degree of individuality, is indicated by the 
fact that it presents so very few modifications. The mucus-, 
pus-, and lymph-corpuscles are small round cells with a nucleus 
attached to their walls. According to Henle, mucus- and 
pus-corpuscles cannot be distinguished in any way from one 
another, and those of lymph differ from them only imasmuch 
as their nucleus is more round and granulous, and does not 
crumble under the action of acetic acid. No difference exists 
between them in the form of the entire cell. The blood-cor- 
puscles present a higher degree of development in this class. 
In them we not only find very characteristic cell-contents, 
the red colourimg matter, but the form of the cell also under- 
goes an important alteration, inasmuch as it becomes flattened. 
As this flattening takes place in cells which float free in a 
fluid, it cannot be explained as the result of mechanical causes, 
but must manifestly be regarded as a peculiar stage of deve- 
lopment of these cells. The nucleus is persistent in all these 
cells, whilst in those more highly developed it usually disappears 
at some subsequent period. Throughout this class the cyto- 
blastema is a fluid; and it is present in greater quantity than 
we shall find to be the case in the next class. If the egg be 
included in this class, we have yet another peculiarity in the 
cells to be added to the above; viz. that not only have the 
separate yelk-cells cell-contents consisting of distinct granules, 
but that the development of the yelk-cells within the yelk 
considered as one cell, is a formation of cells within cells, and 
in some of these cells even a second enclosure takes place. 
This peculiarity, however, is one which may almost be said to 
stand in inverse ratio to the importance of the tissue. It 
is most frequent, perhaps indeed universal, in vegetables, 
occurs more rarely in animals, as in the egg, crystalline lens, 
cartilage, and so on, and appears to be altogether absent in 
the higher structures, as areolar tissue, muscle, &c. We 
have already discussed the other peculiarities of the cells of the 
egg. In the following class we shall not only find a greater 
change in the form of the cells. from flattening, but we shall 
also become acquainted with many other different modifica- 
tions of them. 


INDEPENDENT CELLS, ETC. 73 


CLASS IT. 
Independent Cells united into continuous Tissues. 


This class presents us with the greatest similarity between 
animal and vegetable structure, and, indeed, in so high a 
degree, that even an experienced botanist cannot distinguish 
some of the objects which belong to it from vegetable tissue. 
Most animal cells may be distinguished from the mature vege- 
table cells by thei greater softness and delicacy ; but those 
characteristics are in some measure wanting in this class, and it 
would be very difficult to distinguish microscopically between a 
thin layer cut off from the interior of the shaft of a feather and 
a portion of vegetable tissue. We shall, therefore, take the 
feather as our example, and endeavour to trace these cells, 
which correspond in so striking a manner with vegetable tissue, 
backwards to their primitive condition, explaining this transi- 
tion by delineations, and in this way convince ourselves that, 
in their early stage, they also accord with the primitive cells 
of all other tissues. The tissues comprised under the term 
horny belong to this class, and the crystalline lens may also 
be included in it. The cells of these tissues generally remain 
independent, but more or less intimate blendings of the cell-walls 
with one another also occur in this class. Horny tissue may be 
reduced to two unessential subdivisions, viz.m—l. Its mem- 
branous expansions, to which belong the Epithelium, in the 
extended sense of the term (including the Epidermis), and the 
Pigmentum nigrum, which must be enumerated here, in con- 
sequence of its intimate alliance with the epithelium. 2. The 
compact horny formations, including the Nails, Claws, Hair, 
Feathers, &c. 


1. Epithelium.—lt is very difficult to determine what this 
term ought to comprise. The cortical substance of the chorda 
dorsalis, which is composed of flattened hexagonal cells (in the 
larva of Rana esculenta, for example), cannot be regarded as 
epithelium, since it is made up of the same cells as those of the 
interior of the chorda dorsalis : the sole difference consisting in 


74 EPITHELIUM. 


their being flattened. The serous layer of the germinal mem- 
brane also cannot well be considered to be epithelium, although 
it has the same structure, and yet it is difficult to give a defini- 
tion of it which shall not comprise these structures. We shall 
not, however, enter upon this contention about mere terms, but 
proceed to the consideration of the structure of the epithelium. 

The simplest form of epithelium is that of the round cells 
furnished with a nucleus which lies upon the inner surface of 
their wall, and encloses one or two nucleoli. When in con- 
nexion they assume a polyhedral form, but their free surface 
usually projects in the form of a segment of a sphere. Such 
is the appearance presented by the epithelium in many situa- 
tions; I instance only that of the branchial rays of the fish 
by way of illustration. The cells are usually smaller and more 
eranulous in mammalia; but in the lower animals and in the 
foetal stage of mammalia they are, in general, larger, smoother, 
and sometimes so transparent as to be visible by a subdued 
light only. I once had an excellent opportunity of observing 
the epithelium upon the mucous membrane of the stomach of 
a foetal sheep, and its perfect resemblance to the parenchy- 
matous cellular tissue of plants. A minutely granulous deposit 
may often be observed in the interior of the transparent epi- 
thelial cells; im those of the branchial rays of the fish, for 
instance, it appears to be formed in the neighbourhood of the 
nucleus. According to Henle, two nuclei never occur in an 
epithelial cell in mammalia; but I have several times observed 
that number in the external covering of the tadpole, and on one 
occasion | remarked that a perfectly developed epithelial cell 
furnished with a nucleus was enclosed within a larger cell. 
Changes in form from this rudimentary globular shape occur in 
the epithelial cells in two different manners ; they either become 
flattened into tables, or prolonged into cylinders. The flattening 
out into tables takes place in such a manner that the nucleus 
forms the centre of one surface, as in the blood-corpuscle. I 
have observed the stages of transition from the globular to the 
tabular form in the epithelium of the external covering of the 
tadpole, which occasionally presented hexagonal flat columns 
or tables, the thickness of which was about equal to one third 
of their breadth. The thickness is so very slight m proportion 
to the breadth in the completely flattened epithehal cells, that 


Siew) (Pee Y 


ee ee 


we ie 


fad 

¢ 
' 
ba 


EPITHELIUM. 79 


itis no longer possible to distinguish the two lamellee of the cell- 
membrane. It often occurs that the tabular epithelial cells 
are not regularly hexagonal, but represent flat elongated stripes, 
a fact which has been observed by Henle in the epithelium of 
the vessels." The cells which are prolonged into cylinders con- 


' During several years past I have occasionally observed an innermost apparently 
structureless layer in different parts of the vessels, and as the elastic fibres of the 
middle coat of arteries become gradually more and more minute towards the interior 
of the vessel, and at length are scarcely perceptible, I regarded the layer above de- 
scribed as analogous to the middle arterial coat, in every respect but the possibility 
of discovering fibres in it. I explained certain scattered spots which occurred in 
it, by analogy with the middle and external coats of vessels. Lamelle, for instance, 
were occasionally present, in which the elastic fibres had coalesced more or less 
intimately, and only a trace of a fibrous arrangement remained. In such instances 
there is seen a table composed of elastic tissue, perforated at different spots; I 
regarded those spots as openings which might perhaps be filled with some foreign 
substance. Purkinje and Rauschel (de Arter. et Venar. Structura) acknowledged 
the accordance of this membrane with the middle arterial coat, but distinguished it 
as a separate layer. Valentin denied that accordance, and described it as a peculiar 
structureless membrane. Henle was the first to explain its true relations. By his 
mode of scraping the internal surface of the vessels he obtained scales, which, 
from our present more accurate knowledge, we now recognise as epithelium. They 
were sometimes converted into lamella. There cannot in fact be a doubt about the 
correctness of this explanation, when the vessels of the fwetus are examined. I 
obtained by scraping, both from the larger veins and heart of a feetal pig, large 
lamellae of the most beautiful epithelium, consisting of flat stripes, which were nearly 
as long again as broad, and contained a very distinct and, in proportion to the size 
of the scales, large nucleus, with one or two nucleoli. I could not succeed so well 
in the few attempts which I made on arteries; probably the scales separate more 
readily from one another in them, and can then no longer be distinguished from the 
primitive cells of the elastic coat. The cells probably coalesce more or less in- 
timately at a subsequent period, so as to form what is then a partially structureless 
layer, and the nuclei also disappear in part. I now conjecture that the above- 
described spots upon the inner coat may probably be persistent nuclei; I have not, 
however, made any new investigation upon the subject. With respect to the situation 
in which the one or other form of epithelium occurs, I refer to Henle’s very complete 
treatise (Miiller’s Archiv, 1838, Heft 1). In addition to the parts mentioned by 
Henle, I have found epithelium upon the internal surface of the amnion in the foetus 
of mammalia and man, where the hexagonal scales were very large and beautiful, 
enclosing a very distinct nucleus and nucleolus. Amongst those in the feetal pig 
were some larger round cells, furnished with a larger nucleus without a nucleolus. 
The inner surface of the portion of the allantois projecting from the chorion in the 
same foetus was also lined with tesselated (tabular, scaly) epithelium consisting of 
small scales. The external surface of the chorion was formed of cylindrical cells 
closely packed together, and provided with a nucleus, being similar to the epithelial 
cylinders of the intestinal mucous membrane discovered by Henle. 


76 EPITHELIUM. 


stitute the other modification in the form of the epithelial cells. 
They were discovered by Henle in the intestinal mucus-mem- 
brane. Thev likewise enclose the characteristic nucleus, and are 
arranged with their longest sides in apposition. Their blunt 
ends are turned outwards and free. The opposite end either 
terminates abruptly also, as in the chorion, or proceeds to a 
point. This tapering figure frequently commences at the upper 
part, so that the cells then have the form of a poimted cone, 
the base of which is turned towards the outside. Henle found 
that the cilia stand upon the free surfaces of the epithelial 
cylinders in those membranes which present the phenomenon of 
ciliary motion, a fact of itself sufficient to show that the epithe- 
lium ought not to be regarded as a mere inanimate covering 
to the organized structures. 

With regard to the formation of the epithelial cells, Henle 
has already proved the rete Malpighii to consist of round 
nucleated cells, probably the young epidermal cells, and also 
that the diameter of the cells increases towards the outside, so 
that in the foetal pig he was enabled to trace the gradual transi- 
tion of the cells of the rete Malpighii into those of the epidermis. 
(Symbol ad anatomiam villor. intest., p.5.) An actual growth 
of the epithelial cells thus became very probable ; I have likewise 
followed this process in the foetal pig. The uppermost layer of 
the epidermis is there formed of large, tabular, hexagonal cells, 
furnished with a nucleus. Immediately beneath these le 
nucleated cells, which are already much smaller, and almost 
round, so that the flattening must take place very rapidly. The 
farther you proceed from the surface the smaller the cells be- 
come, and the closer they encompass the nucleus. The size 
of the nucleus also diminishes in some degree, but by no means 
in the same proportion. In the lowest strata, the cells cannot 
any longer be distinguished, but the nuclei le close together, 
with a small quantity of minutely granulous intermediate sub- 
stance. It is, however, very difficult to obtain positive convic- 
tion of this fact, for the stratum of nuclei is too firmly connected 
with the cutis. We shall have an opportunity of observing 
this relation of the nuclei more distinctly hereafter in the 
feather. The mode of formation is probably this : cell-nuclei are 
formed, in the first place, immediately upon the surface of the 
cutis; and then around, and closely encompassing them, the cells. 


PIGMENT. 77 


The cells and the nuclei (the latter, however, in a much less 
proportion) increase in size, and at length those in the upper- 
most layers become flattened in such a manner that the nucleus 
forms the centre of the table. This, then, is but a repetition 
of the same course of development observed in most other cells. 
Before I had proved the universal accordance between animal 
and vegetable cells, Henle thought that the original increase 
in volume of the epithelial cells might possibly be explained as 
taking place by imbibition. (1. c., p. 9.) As, however, we 
have observed this growth to be a phenomenon which occurs in 
all animal cells—as we have seen the formation of cells around 
the nuclei—as a chemical change in the cell-membrane may be 
proved to take place during the expansion of many of the cells, 
and as it frequently happens that not only does no thinning of 
the cell-membrane occur during expansion, but that an actual 
thickening takes place, all which are processes similar to those 
of plants—we must ascribe a peculiar vitality to the animal as 
well as to the vegetable cells, and explain this expansion of the 
epithelial cells, ike as we did that of plants, as a growth by 
intussusception. The new epithelial cells, it is true, are formed 
immediately upon the cutis only, where the greatest vital energy 
prevails; but the cells expand independently, and grow by 
intussusception. I have brought forward an instance in which 
a young epithelial cell was formed within another in the tadpole. 
But this is certainly a very rare circumstance, and the majority 
of epithelial cells, in all the vertebrate animals, are certainly not 
formed as cells within cells, but on the outside of the cells in 
a minimum of cytoblastema, which is exuded from the cutis. It 
might be objected that this process of formation of the epi- 
thelium could not be possible, for the reason that, if the 
cells of the second stratum were twice as large as those of the 
first, then the whole layer of epidermis must be also twice as 
large as the first. But this objection may be easily set aside 
by the fact that the cells slide upon one another, and a double 
or triple layer of cells may thus origimate from one stratum 
of nuclei. 


2. The Pigmentum nigrum. 'The pigment is familiarly known 
as being usually contained in round or (in consequence of their 
close apposition) hexagonal cells, in the form of innumerable 


78 PIGMENT. 


very minute granules, which exhibit a lively molecular motion. 
This motion may sometimes be observed even within the cells, 
so that the rest of their contents must be fluid. As it is also 
known, that the pigment-granules may sometimes be pressed out 
from the cells, no doubt can exist respecting the cellular nature 
of these bodies, formerly called pigment-globules. The wall of 
the pigment-cells exhibits a nucleus, which is already familiar to 
some observers. It may be seen in the foetal condition of the 
pigment cells of the choroid coat in mammalia, at different 
points in that of the very young feetal pig for instance, quite 
distinctly ; and it occasions the well-known white spot in the 
centre of the cells. It commonly contains one or two nucleoli. 
It sometimes happens that no pigment-granules are deposited 
around the nucleus, but that it is surrounded by a clear, trans- 
parent areola. 

Some pigment-cells undergo a most remarkable transforma- 
tion, and one which acquires an especial importance, from the 
fact that it serves as a type of formation for other more im- 
portant classes of cells. This transformation consists in the 
cells being elongated on three or more sides into hollow fibres. 
These we shall name stellated cells. It has, indeed, been 
necessary to allude to them already when treating of bone. 
The characteristic contents of the pigment-cells render them 
best adapted for an accurate examination of this type of for- 
mation. The stellated pigment-cells, known under the name 
pigment-ramifications, are best observed in the skin of the 
tadpole. They exhibit varieties in form; we select for our 
description such of them as present the longest fibres. (See plate 
II, fig. 9.) Their appearance is that of separate black spots, 
from which slender black fibres issue on different sides. The 
black spots represent the bodies of the cells filled with pigment ; 
the fibres are the prolongations of the cells filled with the 
same material. The separate pigment-granules may be dis- 
tinguished in many situations. The body of the cell, which is 
sharply defined on its exterior, sometimes presents a clearer 
spot of a round or oval form, through which the cell-nucleus 
glimmers, and in some few instances can be distinctly per- 
ceived with its nucleolus. The diminution of the cell in various 
directions, in order to pass over into a fibre, is so gradual that 
there is no defined limit between them. The fibres pass be- 


PIGMENT. 79 


tween the cells of the epithelium, and are therefore frequently 
curved : they are in general thickest in the neighbourhood of 
the cell, and diminish as they proceed from it ; but they some- 
times also swell out slightly at some distance from the cell. 
These fibres give off others at different points. The presence 
of the cell-nucleus, and the fact that all the stages of transition 
from indubitable pigment-cells to these bodies may be demon- 
strated, are sufficient evidence that these black spots, with the 
fibres proceeding from them, are actually cells, and that the 
fibres are hollow prolongations of them filled with pigment. 
These transitions are delineated in plate II, fig. 8, just as they 
existed close together in another part of the tail of a tadpole: 
@ is an indubitable pigment-cell, scarcely differing from an 
ordinary one; it has also its nucleus. The only circumstance 
which distinguishes the majority of the primitive cells of these 
stellated pigment-cells from common pigment-cells is that they 
are generally smaller, and more closely filled with pigment. 
6 is a smaller cell, which has commenced to taper; and ¢ is 
distinctly elongated into a fibre. A slightly clearer spot is the 
only indication of the nucleus in both imstances. d and e 
elongate at both ends into fibres, one of which (the upper end 
of d) terminates in a knob witha defined outline. At the spot 
where this knob unites with the body of the cell, a shading, 
indicating a cavity, may be clearly perceived, the pigment 
being more closely deposited in the neighbourhood of the cell- 
wall than in the centre; and lastly, fis a cell which elongates 
into fibres on three sides. When a small piece of the 
skin of the tadpole is torn in water, separate portions of 
these pigment-fibres, or prolongations of the cells filled with 
pigment, may be observed to float about isolated. Instances 
sometimes occur in which one of these pigment-fibres passes 
uninterruptedly from the body of one cell to that of another ; 
for example, fig. 9, a. We may imagine this to be effected 
by the prolongations of two cells meeting at one point. 
As the pigment does not move from one cell to another, we 
cannot accurately determine whether the partition-walls be- 
come absorbed at such a point or not. Such, however, 
may be supposed to be the case, otherwise an interruption of 
the pigment corresponding to the double thickness of the cell- 
wall must be seen at the spot where the prolongations are in 


80 NAILS. 


contact. The fibres issuing from the cells often become very 
minute in the last part of their course, from which we learn 
that the delicacy of fibres does not preclude their being 
hollow. 


3. Nails.—In order to investigate the structure of the nail 
we should make use of that of a child immediately after birth, 
or, what is better, that of a mature, unborn, human foetus ; 
such an one, when divided into delicate longitudinal sec- 
tions, will be found to consist of laminz deposited one upon 
another, surface to surface. This laminated arrangement, how- 
ever, becomes more and more indistinct upon the under surface 
of the nail which lies upon the skin, the nearer we approach to 
that portion contained in the fold of skin at the root, and 
the posterior half of the part which is embedded in that fold 
exhibits no laminated structure whatever, but consists of small 
polyhedral cells, many of which present perfectly distinct cell- 
nuclei. When a small portion is cut or torn off from the 
surface of such a nail, the form of the margins, which present 
smooth angular projections, leads at once to the supposition 
that the lamine of the nail are not structureless, but pro- 
duced by the junction of little scales resembling those of 
epithelium. When treated with acetic or concentrated sul- 
phuric acid, the scales separate more readily, and in some rare 
instances an indistinct nucleus may be recognized in them. 
No such scales can be seen in the root of the nail after the 
adherent lamella of epidermis has been scraped off, but polyhe- 
dral cells, which are much smaller than the scales, are found 
in that situation. Now it is a well known fact that the nail in- 
creases from its root, and is constantly pushed forwards. ‘The 
polyhedral cells of the root must thus, therefore, become 
transformed into those scales by flattening and extension of 
their superficies, a process which the independent vitality of the 
cells renders easily conceivable. ‘The cells of the nail already 
formed increase in size from the same cause, and the growth 
of the nail by no means depends upon a mere apposition at its 
root, although it is probable that the formation of new cells 
takes place in that situation only where the nail is in con- 
nexion with the organized skin. The nail would certainly be 
pushed forwards by the extension of the superficies of those cells, 


HOOEFS. 81 


and by their flattening in reference to its thickness, but the 
more the cells become flattened, the thinner must the anterior 
part of the nail become. ‘This probably is compensated for, 
by a formation of epithelium-scales upon the under surface of 
the nail, and especially at its posterior part. If, for example, 
an epithelium-scale become attached to the most posterior part 
of its under surface, it will be advanced somewhat forwards by 
the flattening of the cells above, and the formation of new 
cells at the end of the nail. At that part, however, a new 
scale is next formed, and laid upon the former one, and as the 
advance forwards goes on, a third and fourth are formed, and 
so on, so that, by this means, a thickening of the nail must take 
place proportionate to its advance from behind forwards. I 
consider, therefore, that this thickening of the nail, in conse- 
quence of growth from the under surface, and the thinning 
consequent upon the flattening of the cells, compensate each 
other, and that the almost uniform thickness of the nail is 
produced by this means. The superficial laminz of that part 
of the nail which lies external to the fold of skin at all events 
do not continue to grow. I marked several nails with two 
points, by boring them with a needle and colouring the spot 
with nitrate of silver; the marks were made at the root of the 
nail, some in the longitudinal, others in the transverse direc- 
‘tion. In the course of two or three months they had advanced 
to the point of the nail, but their distance from each other had 
not altered in the least. 


4. Hoofs. The horny tissue of hoofs, in the foetus at least, 
consists entirely of the most beautiful vegetable-like cells. If 
a thin transverse lamella be cut off from the hoof of a large 
foetal pig, the preparation will present the exact appearance of 
vegetable cellular tissue. The following facts prove that the 
cells are not flat: in the first place, when the side walls do 
not stand quite perpendicular, they may be traced down below 
the level of the section, and the depth to which they go may 
be estimated ; and secondly, longitudinal sections of the horny 
tissue of hoofs present a similar appearance to those made in 
the transverse direction. They are, therefore, polyhedral cells, 
and some of them, at least, contain a distinct nucleus. 

When the tissue is quite fresh, it is not possible to distin- 

6 


82 FEATHERS. 


guish the particular wall of every cell. But when the foetus 
has lain for a time in strong spirit, the horny substance of the 
hoof may be easily separated from the foot, in consequence of 
the connexion between the cells having become looser. The 
undermost layers of cells, however, remain attached to the 
foot. The interior of the layer of horny substance so sepa- 
rated, consists of a crumbling mass, somewhat resembling a 
boiled yelk. The particles cannot, however, be separated quite 
so readily from one another as those of the yelk are. With 
the aid of the microscope, this mass is found to be composed 
of irregularly angular bodies, resembling the yelk-substance 
when boiled. These bodies are the isolated cells, whose pecu- 
har walls are distinctly perceptible, and some few of them 
have a nucleus, which lies upon the inner surface of their 
wall. <A continuous firm layer of flat epithelium-scales, the 
immediate continuation of the outer lamelle of the epidermis, 
consisting of flat cells, surrounds these polyhedral cells as an 
external covering to the entire hoof. This lamella exists in 
the foetal pig at a very early age, the layer of polyhedral 
cells being at that time very slight ; in a more advanced stage 
of development, however, the latter forms the chief mass of the 
horny substance of the hoof. In the recent condition these 
cells must also have somewhat firm contents, otherwise, with 
so delicate a cell-membrane, the substance could not be so 
firm. But its elasticity was such as to prevent my crushing 
one of the cells with the compressorium, my object being to ob- 
serve whether the cell-contents would flow out, or be torn like 
a firm substance. As the cell-contents form a large portion of 
this horny substance, whilst the nails consist for the most part 
of flat cells without any discernible contents, almost entirely 
therefore of cell-walls, a chemical distinction may be presumed 
to exist between the two structures. 


5. Feathers. The feather is composed of the quill, the 
shaft, and the vane, or beard. The elementary structure of 
these parts is, however, what most interests us at present ; and 
in order to investigate it, at least in order to become acquainted 
with the relation which the different elementary formations in 
the feather bear to cells, we must take one in which a part of 
the shaft is in progress of formation. The feathers at that 


FEATHERS. 83 


time are surrounded by a dense capsule, which is composed, 
throughout its entire thickness, of gigantic tabular epithelium. 
The feather is so placed in this capsule, that the shaft and vane 
are folded together to form a hollow cylinder, which is occu- 
pied by the so-called organized matrix of the feather (see an 
article on this subject by Fr. Cuvier, in Froriep’s ‘ Notizen,’ 
No. 317). According to Cuvier, a membrane lines the inner 
surface of the vane, and gives off septa, which penetrate be- 
tween its separate barbs. This membrane, however, as well as 
the septa, is composed of epithelium. 

The shaft of the feather consists of a loose medullary sub- 
stance (the pith), surrounded by a firm cortex. On making 
thin transverse or longitudinal sections of the pith, it is seen 
to be composed of beautiful polyhedral cells, which perfectly 
resemble the parenchymatous cellular tissue of plants,—as the 
substance of cork for example. (See plate II, fig. 10.) The 
cell-cavities which have moderately thick, dark partition-walls, 
are at first filled with a transparent fluid, but subsequently 
become dry, and in that state contain air. Notwithstanding, 
however, that this pith so precisely resembles vegetable tissue 
in its general appearance, it may be questioned whether these 
cells be actually cells in that sense of the word in which we 
receive it here, viz. elementary cells of organic structure, and 
whether they correspond to vegetable cells. It therefore be- 
comes necessary to investigate whether each cell has its pecu- 
liar wall, and whether the course of development of each in- 
dividual cell be the same as in plants. There is no structure, 
however, in which it is easier to follow the process of develop- 
ment than in the one before us, chiefly because the cells, 
even from the first, have no connexion with the organized 
so-called matrix, but remain attached to the fully-developed 
cells of the shaft, when the matrix, which terminates ex- 
ternally with a smooth surface, is taken away. The following 
description is taken from the large wing-feather of a raven: 
it applies however equally well to the feathers of the young 
chicken. 

The pith, when in progress of formation, is soft and friable. 
When a small portion of it is examined, after the component 
particles have been separated asunder, it is found to consist 
of cells, in various stages of development. Those which 


84 FEATHERS. 


are most completely developed resemble the cells which we 
have seen in the mature feather (see plate II, fig. 11, a) 
in every other respect, but that they lie less firmly connected 
together, so that they may readily be isolated, and the peculiar 
wall of each cell be distinctly seen. The walls are of sufhi- 
cient thickness to prevent their losing their angular shape, 
even when in the isolated state. There are intercellular spaces 
between some of them, and such also occur between the fully- 
developed cells of the perfect feather. The cell-membrane is 
dark and smooth, and the cell-contents consist of a transparent 
fluid. A very distinct nucleus is also seen lying upon the wall 
of each cell. It is oval, and contains one or two nucleoli, 
which are large in proportion to its size (see the figure). 
There is no nucleus to be seen in the fully-developed cells, 
and it is only in very rare instances that its remains can be 
detected ; it must therefore undergo absorption at some sub- 
sequent period. ‘The process may, indeed, be followed; for 
example, the cells which form the stages of transition between 
those delineated in fig. 11 a, and fig. 10, are more closely 
connected together, the nucleus in them becomes smaller, 
and its outlme more irregular, the nucleolus meanwhile re- 
mains; at length both disappear. The degree of development 
attained by the cells is generally proportionate to their dis- 
tance from the matrix ; and as that is situated on the inner 
side of the feather, at the part where the shaft exhibits a 
furrow, those cells which lie immediately under the cortex, at 
the back of the shaft, are the most perfectly developed. Now 
the cells when traced from that part inwards towards the ma- 
trix, are found to become gradually smaller; the cell-mem- 
branes lose their dark outlines, and present a granulous aspect. 
The nucleus of the larger granulous cells has still the same 
form as in those previously described with a smooth cell-mem- 
brane; its size, however, diminishes with that of the cell. 
The cells in this granulous condition resemble most of the 
elementary cells of other tissues. Plate II, fig. 11, 4, c, repre- 
sents the stages of transition. Advancing still nearer towards 
the matrix, the cells are no longer recognizable; all that we 
can see being numerous nuclei, which le close together in a 
minutely-granulous substance (plate II, fig. 12). 

The process of formation of the cells of the pith is, there. 


FEATHERS. 85 


fore, as follows: a minutely-granulous mass is present in the 
first instance, in which numerous cell-nuclei lice, some of them 
exhibiting a nucleolus. Around them the cells are formed, 
being at first not much larger than the nuclei, and having a 
granulous aspect. The cells gradually expand; the nucleus 
also grows, and soon reaches its full maturity. It remains 
eccentrical, lying upon the cell-wall. The cell-membrane re- 
tas its granulous aspect for a time; gradually losing it, 
however, as the expansion of the cells advances; at the same 
time the contents of the cell-membrane become darker, but the 
cell-walls are not at all diminished in thickness. ‘The walis 
of the cells, im the next place, become more firmly united 
together, so that they cannot be separated from one another 
so readily, and at the same time the nucleus gradually dis- 
appears. The contents of the cells at last dry up, and they 
become filled with air. The development of these cells ac- 
cords, therefore, entirely with the vegetable cells, the nucleus 
being their true cytoblast ; it is present before the cell, and, 
as is generally the case im the cells of plants, afterwards be- 
comes absorbed. The cell expands, growing by intussuscep- 
tion, and the membrane of the fully-developed cell might, 
without much danger of error, be assumed to be more than 
ten times heavier than that of the youngest one. The phy- 
sical, and probably also the chemical, condition of the cell- 
membrane undergoes a change. The cytoblastema, in which 
the cell-nuclei are in the first place formed, consists of gra- 
nules, analogous to the mucus-granules, in which, according 
to Schleiden (Miiller’s Archiv, 1838, plate III, fig. 2), the 
cytoblasts of vegetable cells origimate. According to Schleiden, 
those mucus-granules are deposited from a solution of gum 
within a parent-cell. The cells of feathers are not formed in 
parent-cells, but in the neighbourhood of the organized matrix. 
There can be no doubt, however, but that the matrix only 
exudes a fluid, which afterwards becomes transformed into a 
granulous substance. I have not investigated the mode in 
which the nuclei originate in the cytoblastema, whether 
by a junction of smaller globules, whether the nucleoli first 
exist, and so forth. The growth of the nucleus proceeds for 
a time with that of the cell; for the latter is formed around 
the nucleus before it has reached its full size. The cytoblas- 


86 FEATHERS. 


tema of the cells of the pith of the feather is supplied by the 
nearest contiguous substance provided with vessels, that is, by 
the so-called matrix. In the young feathers of the hen, how- 
ever, [ found a layer of very small, extremely pale, round cells 
without nuclei,—a sort of imperfect epithelium,—between 
the matrix and the granulous cytoblastema, so that not even 
so much as an immediate contact exists between the latter and 
the organized substance. 

The cortical substance of the shaft of the feather is a 
fibrous structure. Here the Cell-theory seems, at first sight, 
to fail; but we are soon taught otherwise, when we examine | 
the generation of the fibres as exhibited in the completely 
formed portion of the cortical substance of a feather, which 
is in progress of formation within the capsule. The cortex 
is then seen to consist of large flat epithelium-cells, each 
having a beautiful nucleus, with one or sometimes two 
nucleoli. Some of these epithelial tables are long fiat stripes, 
others are of an irregular rhomboidal form. They are very 
firmly connected together. Hach cell generates several fibres, 
and the transitions may be readily observed at different 
parts of the same preparation. Plate II, fig. 13, represents 
them. The cells at first are flat tables, having a smooth 
margin, a slightly granulous aspect, and containing a very dis- 
tinct nucleus (fig. 13, a). Upon their margins and sur- 
faces indistinct fibres gradually become visible, which project 
out insulated from the edges, but are connected together upon 
the surface by the substance of the tables (fig. 13, 0). At 
this stage the fibres are pale, and the nucleus of the cell still 
quite visible. ‘The fibres afterwards become more sharply and 
darkly defined; the imsulated portions projecting from the 
edges are larger, the part of the table connecting them together 
becomes more indistinct, and the nucleus begins to wane, 
although it is still distinctly perceptible, and the nucleolus 
especially so (fig. 13, ec). At length all traces of the original 
cell and the nucleus disappear, and we see only dark, stiff, 
thin fibres, which are closely connected together but may 
still be recognized as being insulated for a space, the length of 
the original table (fig. 13, d). These fibres, therefore, also 
originate from cells, and that not so much by an elongation 
of the cells, as by their division into several fibres. As the 


CRYSTALLINE LENS. 87 


fibres which lie close together in the first instance, do not, 
as it seems, continue connected with one another, a portion 
of the original table must be absorbed, and the following 
may therefore be conceived to be the mode in which the 
fibres originate. After the two laminz of the table are in 
part or entirely blended together, an absorption takes place 
at certain parts, in such a manner, that the portions not 
absorbed lie in longitudinal lines, and thus remain as fibres. 
The reality of an absorption is, moreover, distinctly shown by 
the disappearance of the cell-nucleus. We have no evidence 
as to whether the fibres are hollow or not; it is sufficient for 
our purpose to know that they originate by a transformation 
of cells. 

The quill of the feather has a similar structure to that of 
the cortical substance of the shaft. 

The vane is composed of separate barbs, and each barb is 
again a miniature feather. The following description is taken 
from the undeveloped wing-feather of a sparrow. Each barb 
contains a secondary shaft, on the side of which is placed a 
secondary vane. ‘The secondary shaft has the same structure 
as the principal one, and consists of a cellular medullary sub- 
stance (pith), and a firm cortex. The secondary vane is com- 
posed of a great many triangles, which lie with their surfaces 
close together, having very narrow bases by which they are 
fixed upon the secondary shaft. Each triangle is formed of 
flat epithelium-cells arranged with their angles overlapping 
each other, each having its nucleus. The separate epithelium- 
cells are broadest below, diminish more and more towards the 
point, and extend proportionately in length. The nuclei lie 
in a row, near about the middle line of the triangle. The 
last cell, at the apex of the triangle, is contracted into a long 
fibre. The last cell but one, and all the others in succession, 
become elongated, at the poimt at which the next following 
cell is attached to them, into pointed processes, which vary in 
length, and are extended on both sides of the cells in the plane 
of the triangle. 


6. The Crystalline lens. The mode in which the lens is 
nourished has always been an enigma. Having no vessels, it 
has either been regarded as a secretion of its capsule, or its 


88 CRYSTALLINE LENS. 


mode of life has been considered as generally resembling that 
of vegetables. We shall find the latter to be the correct view, 
and the singularity of the mode of its nutrition disappears 
altogether, when we become acquainted with the fact, that the 
growth of the organized tissues resembles that of vegetables. 
The general statement, that the lens has the vitality of a vege- 
table, does not, however, express much, unless the relation of 
its elementary structure to the cells of plants be proved. The 
lens is known to be composed of concentric layers, made up of 
characteristic fibres, which, not to go into details, may be said 
to pursue a general course from the anterior to the posterior 
surface. 

In order to become acquainted with the relation which these 
fibres bear to the elementary cells of organic tissues, we must 
trace their development in the foetus. When the lens of a 
chick is examined after eight days’ incubation of the egg, no 
fibres are to be found; but it is composed of round, extremely 
pale, and transparent smooth cells. Some contain the charac- 
teristic cell-nucleus, in others it cannot be detected ; and there 
are also many nuclei without surrounding cells. Some larger 
cells may be observed in the chick at a more advanced period, 
which contain in their interior one or two smaller ones (see 
pl. I, fig. 10, d, from a foetal pig), and from the manner in which 
these cells become flattened against the wall of the parent-cell, 
as well as from the presence of the nucleus in other cells, we 
may conclude, that these pale globules are actually cells, al- 
though a cell-membrane be not distinctly recognizable. Wer- 
neck, who first observed them, likewise calls them cells. 

The following conditions of the crystalline lens may be ob- 
sorved in Mammalia. In a foetal pig, three and a half imches in 
length, the greater part of the fibres of the lens is already 
formed ; a portion, however, is still incomplete; and there are 
many round cells awaiting their transformation. The perfected 
fibres form a sphere in the centre of the lens; but there is no 
laminated structure as yet perceptible in it. The fibres may 
readily be separated from each other, and proceed in an arched 
form from the anterior towards the posterior side of the lens. 
This sphere, composed of the perfected fibres, becomes sur- 
rounded, in the circumference of the lens, with a thick and 
broad zone of fibres, which are as yet imperfectly developed. 


_— 


CRYSTALLINE LENS. 89 


They have much the same course as the others, that is, they 
form arches from the anterior towards the posterior surface. 
They do not, however, reach the axis either in front or behind, 
but the fibrous zone is thickest in the middle, gradually dimi- 
nishes towards the anterior and posterior surfaces of the lens, 
and terminates altogether without the fibres meeting anywhere 
in front, or reaching the axis. No laminated structure can 
be perceived in the zone; but the fibres may be readily imsu- 
lated throughout its entire breadth. When the ends of these 
fibres are examined, they are found to be either simply rounded 
off, or to terminate in a small round dilatation, or to pass over 
into larger globules (cells) ; or, on the contrary, it may be more 
correctly expressed by saying, that the larger globules or cells 
become elongated to these fibres (see pl. I, fig. 12). The 
transition from cells to fibres may either be very gradual or 
somewhat sudden ; but even in the latter case, the contour of 
the cell passes immediately over into that of the fibre, so that 
the latter is not merely affixed to the globule, but is a true 
continuation of it. Now, these cells which become elongated 
into fibres, perfectly accord with other neighbouring cells which 
are as yet quite round; and these again accord with the cells 
forming the greater portion of the lens in the embryo chick. 
They are round, extremely pale, smooth, transparent cells of 
very various size (see pl. I, fig. 10). Some have a very beau- 
tiful, sharply-defined, oval nucleus, which, in most instances, is 
not flattened, and which lies upon their wall, and encloses one 
or two nucleoli. Some cells are scarcely larger than the nu- 
cleus which they contain, fig. 10, 5, for example. Some of 
these enclose young cells (fig. 10, d), and as they may be ob- 
served to flatten against the wall of the parent-cell, there would 
seem to be no question about the existence of a special cell- 
membrane for the latter, and thus the true cellular nature of 
these globules appears indubitable. The presence of the nu- 
cleus, and the fact of the outlines of the cells being too sharply 
defined for mere shadows, would, however, have been sufficient 
to render their cellular character probable. The very distinct 
nucleoli contained in the nuclei, which are not flattened, lie 
upon the inner surface of the wall and not in the centre, as 
represented in fig. 11. 

Since, then, the round cells, as we have seen in the chick, 


90 CRYSTALLINE LENS. 


form the primary structure of the crystallie lens, and no 
fibres can be detected in the early stage, and since the more 
fully-developed lens of the foetal pig exhibited many fibres and 
fewer round cells, and at the same time cells which became 
elongated into fibres, we cannot but regard the fibres gene- 
rally as elongated cells. It is true that a cell-membrane 
cannot be distinguished on the fibres, nor can it be distinctly 
recognized on the round cells. If, however, the arguments 
above cited rendered its existence in the round cells certain, 
they must avail equally in the case of the fibres. Nuclei 
are also frequently found upon the fibres of the foetal pig. 
Some of the fibres are flat. I have, also, several times ob- 
served an arrangement of the nuclei in rows; but I do not 
know what signification to attach to the fact. A blending of 
several cells to form a fibre may also possibly occur; but I 
have no observations decisive of the pomt. In fishes also, 
in a young pike for instance, the elongation of the cells into 
fibres may often be very distinctly seen. 

Brewster found that many fibres of the crystalline lens, espe- 
cially in fishes, exhibit the remarkable peculiarity of having 
their margins serrated. PI. I, fig. 18, represents such a fibre 
taken from the innermost lamina of the lens of a pike. The 
fibres are flat, and their sharp margins furnished with long 
teeth, which are so disposed, that two neighbouring fibres lock 
into each other by them. We have here an instance of perfect 
analogy to a form of vegetable cells, which is delineated in fig. 
14: it is an epidermal cell of a species of grass. It is very 
much elongated, quite flat, and furnished on the sides with 
teeth precisely similar to those of the fibres of the lens, which, 
in like manner, fit in between the denticulations of the con- 
tiguous cells. The fibre-cells of the crystalline lens which are 
delineated, have somewhat longer teeth in comparison with the 
breadth of the cell; they represent, however, some of the most 
strongly denticulated fibres. On pursuing the examination 
from the external towards the internal lamine, the same lens 
will be found to present all possible stages of transition in this 
serration, from the smooth or only minutely-notched cells, 
to such as are strongly denticulated like those in the figure. 
This striking accordance of so remarkable a form of animal 
structure with a similar modification of vegetable cells, is a 


INDEPENDENT CELLS, ETC. 91 


brilliant confirmation of the correctness of the view, that the 
fibres of the crystalline lens are really cells, however much 
they may deviate from the fundamental type of the cellular 
form. 

There is no longer, therefore, any more difficulty in explain- 
ing the process of nutrition in the lens, than there is that of 
plants. The cells grow by their own independent force, and 
blood-vessels are unnecessary, as the nutrient fluid can be con- 
ducted from one cell into another. A morbid change of the 
cell-yitality, rendering the cell-contents opaque, is also possible. 


The structures included in this class, notwithstanding the 
strong general resemblance which they bear to each other, have 
furnished us with far more varied modifications of the cellular 
form than the previous class exhibited ; mdeed, these so-called 
unorganized tissues have already prefigured the type of all the 
changes by which the organized tissues are developed from sim- 
ple cells. Here, also, the fundamental form of the cells is that 
of a sphere, which, in consequence of their close contact, 
passes over, from mechanical causes, into a polyhedral figure. 
Two different modifications of this fundamental form occur, 
which cannot be explained mechanically ; they are the flatten- 
ing of the cells on two opposite sides to form tables, and their 
elongation in two directions into cylinders or fibres. We have 
already seen an instance of flattening of the cells in the blood- 
corpuscles of the previous class. It is not only more strongly 
marked here in the tabular epithelium, where the cell-cavity is 
quite obliterated, but a modification even of this form is pre- 
sented to us in the elongation of these tables on two sides into 
flat stripes, as seen in the epithelium of the internal coat of veins 
for example, and still more distinctly in the cortical substance 
of the shaft of the raven’s feather. The epithelium of many of 
the mucous membranes, that of the intestine for instance, which 
Henle describes as consisting of little palisade-like cylinders 
placed close to one another, furnishes us with a rudimentary 
form of the elongation of cells into cylinders and fibres. 
Sometimes these little cylinders become acuminated at their 
lower extremity, or they may diminish throughout their entire 
length from above downwards, and thus become small cones. 


92 INDEPENDENT CELLS UNITED 


This prolongation of cells into long cylinders (called fibres) 
is much more remarkable in the crystalline lens. The fibres 
or cylindrical cells of the lens, however, themselves undergo 
very important modifications, inasmuch as they often become 
flattened on two sides into bands, and then the margins of 
these bands become denticulated. This serration is probably 
produced by a more forcible expansion, and therefore bulging- 
out of the walls of these bands at different points, which follow 
each other at pretty regular distances, whilst the intervening 
points, situated close to them, remain stationary. All the dif- 
ferent stages of this serration, may be observed in the lens of 
the fish, if the fibres are examined from the exterior towards 
the centre of the structure. Now, in this flat and serrated 
condition, the cells of the crystalline lens perfectly resemble 
those of the epidermis of some grasses, and this accordance 
with indubitable vegetable cells is a proof that, despite the 
modifications which they undergo, they do not lose their 
cellular character. If the explanation I have given of the 
mode in which the serration is produced be correct, it will not 
materially differ in principle from the elongation of the cells 
into cylinders and fibres. For, in the latter case, a more 
forcible expansion of the cells is likewise presumed to take 
place in certain situations : the sole difference being, that in 
the latter case it takes place only at one or two opposite points 
of a cell, whereas with the serration if occurs at many sepa- 
rate ones. At this stage of our inquiry, we are reminded of 
the form of many pigment-cells, in which this expansion of the 
cell, at certain spots, takes place on several sides, and in a far 
higher degree, causing the cell to assume an irregular stellated 
form. The prolongations of these cells, however, retain their 
character as hollow processes, even when almost as minute as 
the fibres of cellular (areolar) tissue. 

The distinction between cell-membrane and cell-contents is 
nowhere more distinctly defined than in the fully-developed 
cells of this class. In the perfected cells of the pith of 
feathers, for example, it is as marked as we ever find it to 
be in plants. When traced backwards to their earliest stages 
of development, their true cellular formation scarcely admits 
of a doubt, although the cell-membrane, for reasons given at 
page 36, cannot be so clearly distinguished. The elementary 


INTO CONTINUOUS TISSUES. 93 


cells of the tissues of the following classes, in most instances, 
do not advance beyond this early stage in the development of 
the feather-cells, but the changes necessary to the formation 
of the subsequent tissues occur at this period; their cellular 
nature is, however, quite as certain as is that of the young 
feather-cells, although it be not possible to recognize their cell- 
wall so clearly as in their perfectly-developed condition. 

The matter contained in the cells is either a transparent 
fluid, as in the cells of the pith of feathers previously to their 
becoming dry, or in the crystalline lens, when it contains 
albumen ; or, a minutely-granulous mass, as in many epithelium- 
cells, or pigment-granules ; or, it is altogether absent, and the 
cell-walls, im consequence of their flattening, are in immediate 
contact. The air in the cells of the pith of mature feathers 
simply penetrates from without, durmg the process of their 
desiccation. With the exception of some of the cells of the lens, 
- all the cells of this class are invariably furnished with a nucleus 
of the characteristic form. It is not, however, a persistent 
structure, as in the previous class, but in very many instances 
becomes absorbed when the cells have reached maturity ; such 
is the case in the pith of the feather, the superior laminz of 
the epidermis, the nails, crystalline lens, &c. &c. 

As a general rule the cells remain independent during all 
these changes, that is to say, each cell retains its especial wall, 
and its own peculiar closed cavity. More or less complete 
blendings of the cell-walls, and even of their cavities also, 
occur, however, as exceptions even in this class. The epithelial 
scales of the nail are so intimately connected together, that it 
is rarely possible to trace the contour of one of them in its 
entire circumference; and the same appears to be the case 
with the epithelium in the vessels of the adult. The coalescence, 
however, does not appear to be perfect, for, by the employment 
of concentrated acids, the scales of the nail may be separated 
somewhat more readily from each other. A union of the 
cavities of several cells seems to occur in the pigment-cells. 
A prolongation of a cell filled with pigment may be seen to 
pass uninterruptedly to the cavity of another cell (plate II, 
fig. 9, a). In such an instance, probably, the prolongations of 
two cell-cavities join at a certain point, the cell-walls unite 
together there, and the partition-wall becomes absorbed, and 


94 INDEPENDENT CELLS UNITED 


thus an uninterrupted passage from one cell-cavity into the 
other is produced. I am not certain as to whether a similar 
process does not take place in some fibres of the crystalline 
lens. 

The transformations which the cells undergo are not, how- 
ever, restricted to those already mentioned. A completely 
opposite process occurs in the cortical substance of the shaft 
of feathers, viz. a division of the cells into fibres. By this 
process, out of a single celi several fibres are generated, which, 
in the first instance, are united together by the rest of the 
substance of the cell, but at a later period of development may 
be insulated to a considerable extent. An elongation of the 
cells into these fibres takes place, indeed, at the same time, 
but the major portion of each fibre is formed by the division 
of the bodies of the cells. 

With respect to the formation of the cells of this class, we 
find it to be a constant rule, that their size increases in pro- 
portion with their age, a fact which Henle has already pointed 
out with. regard to the epithelium. We have seen in the dif- 
ferent tissues, that the nucleus is first present, that the cell is 
then formed around it, the nucleus, therefore, being the true 
cytoblast, and that it holds the same position in these cells 
that it does in those of plants, being fixed eccentrically upon the 
internal surface of the wall. Cell and nucleus advance in 
growth for a time, the former, however, much more vigorously 
than the latter. The nucleus is generally absorbed after the 
formation of the cell is completed. The generation and growth 
of the cells and all the phenomena connected with the nucleus 
resemble those of the vegetable cells, and we may unhesi- 
tatingly draw a parallel between them. In no class is the 
quantity of the cytobiastema smaller than in this. In the 
immature state the walls of the cells lie close together, with 
at the most, but a minimum of intercellular, substance be- 
tween them at points where three cells are in contact, and it 
is only between those nuclei, around which no cells have as yet 
formed, that a somewhat larger quantity of cytoblastema is 
present. . 

The class of cells now treated of, and the teeth which will 
be examined in the following class, have been comprised under 
the term unorganized tissues, and their growth represented as 


INTO CONTINUOUS TISSUES. 95 


dependent upon a secretion of the so-called matrix. If by 
this it is meant that the substance of horn is secreted by the 
matrix and hardened in the air, the view is manifestly an 
erroneous one; what we call horny substance being either 
merely the cell-walls, when, for example, the cells are flat, and 
there are no cell-contents, or the cell-walls and cell-contents 
together, when the cells are polyhedral, as in hoofs. All these 
cells are independent structures, which grow organically. But 
if, by the above description, it is meant that the organized 
matrix only furnishes (or secretes) the cytoblastema, no im- 
portant objection can be raised. The cells of the horny tissue 
require a nutritive fluid for their growth. This is supplied to 
them by the blood, as it is in all tissues. As, however, the 
blood-vessels themselves do not pass between the cells of the 
horny tissue, the nutritive fluid must be furnished by the 
nearest substance in which blood-vessels exist, and in this 
sense the nearest organized substance may be called, matrix. 
But whether this cytoblastema which exudes from the matrix 
have a specific character, and on that account horn-cells are 
formed in it—or whether their formation take place in it for 
the same reason that the muscle-cells, those of areolar tissue, 
and so on, originate in other parts of the body, that is to say, 
whether it is determined by the plan of the entire organism, 
—is a question which does not as yet admit of a decision. It 
is, however, a characteristic of all the cells of this class (with 
the exception of the crystalline lens, which I have not examined 
in reference to the point), that the new cells are not generated 
between those already formed, but only in the cytoblastema 
nearest to the organized substance, if not, indeed, always in 
immediate contact with it. The teeth were necessarily sepa- 
rated from this class, because, as we shall see hereafter they 
present quite a different relation of the cells. The new cells 
of cartilage, so long as it does not contain any vessels, are not 
only formed upon the surface of the tissue, but also between 
the most recently-formed cells. 

The chorda dorsalis forms the transition from this class to 
the following one. The cell-walls remain separate in the 
highest stage of their development, and it is only in their 
rudimentary forms, in the osseous fishes for example, that they 


96 COALESCENCE OF THE CELL-WALLS ONLY. 


coalesce and exhibit fibres between the cell-cavities. It does 
not appear to possess any vessels. The formation of new cells 
goes on at the extremities, for instance, at the point of the 
tail of the tadpole; it is not, however, limited to the surface, 
but appears to take place between the most recently-formed 
cells, for cytoblasts may be observed in the intercellular sub- 
stance between the cells which have reached maturity. In 
this respect the chorda dorsalis resembles cartilage, but differs 
again from it, in that, as Miller discovered, it undergoes no 
change in boiling water, and also, in that, the nuclei are flat, 
while those of cartilage-cells are round or elliptical. 

If the chorda dorsalis be reckoned in this class, it affords, 
as we have seen, an example of the generation of cells within 
cells. A different signification might, however, be ascribed 
to these young cells within the true cells of the chorda dorsalis, 
for they do not seem to be formed like their parent-cells, 
from cytoblasts. A generation of cells within cells takes 
place also in the lens. In all the other tissues of this class, 
with few exceptions, the formation of new cells takes place 
only on the outside of those already existing. 


CLASS III. 


Tissues, in which the cell-walls have coalesced with each other, 
or with the intercellular substance. 


This class comprises the firmest structures of the animal 
body, namely, cartilage, bone, and the ivory and osseous sub- 
stance of the teeth. The following is the type of these tissues 
in their mature state: they present either a multitude of small 
roundish cavities in a firm transparent substance, or cavities, 
from which canaliculi issue out in a stellate form; or again, 
merely canaliculi dispersed through the: tissue with tolerable 
regularity. The cavities do not communicate immediately 
with each other; the canaliculi, however, often unite together. 
A special cell-membrane cannot be distinguished in any of 
them in the mature condition, but in an earler stage the 
cavities may be proved to be cells, that is, hollow spaces en- 


THE TEETH. 97 


closed by a peculiar membrane, and the canaliculi are then also 
seen to be hollow prolongations of cells. The intermediate 
substance between the cavities is produced in one of the fol- 
lowing ways: either the walls of the cells become thickened, 
and then coalesce to form an homogeneous substance, or, which 
is much the more frequent mode, the intercellular substance 
is developed in greater quantity, and a coalescence takes place 
between it and the unthickened or only slightly thickened 
cell-walls. I cannot positively assert that a blending of the un- 
thickened cell-walls with the intercellular substance takes place 
universally : I cannot do so, for instance, with respect to the 
cartilages of the higher animals, and so far the mere coalescence 
of the cell-walls is not a certain characteristic of this class 
of tissues. Should it be found not to prevail universally we 
must look for a distinctive character in the abundant develop- 
ment of a firm intercellular substance—a peculiarity which 
is presented by no other class. . 


1. Cartilage and Bone. As these tissues have been already 
treated of (pp. 15-33), the reader is referred to that part of the 
work. 


2. The Teeth. The teeth were formerly classed with the 
bones, but have of late been treated of as non-vascular struc- 
tures under the head of horny tissues. Since Miescher’s 
discovery, however, that the vessels of bone also traverse 
only the medullary canaliculi, since Miiller observed that 
the teeth, like the bones, afford gelatine by boiling, and 
Retzius discovered osseous corpuscles in the ivory, it seems 
more correct to class the teeth with the bones again, and 
the more so, as we now know that the presence or absence 
of vessels proves no essential difference in the growth. The 
coalescence of the cell-walls which appears to take place in the 
ivory of the teeth forms an additional reason for our classing 
them with bone. The teeth, as is well known, consist of ivory, 
osseous substance, and enamel. : 


98 ENAMEL OF THE TEETH. 


a. The enamel. 


The enamel consists, according to Purkinje, of square, or, 
according to Retzius, of hexagonal closely-aggregated prisms, 
which stand nearly perpendicular upon the surface of the ivory, 
and pass outwards in a slightly curved direction. It is at first 
soft, and if some of it be scratched off in that state, we obtain, 
what Miiller has described as needle-shaped bodies pointed at both 
extremities. According to Purkinje, Raschkow, and Retzius, 
some organic substance remains after the young enamel has 
been treated with hydrochloric acid, whilst Berzelius asserts 
that the enamel of mature teeth does not contain two per 
cent. of organic matter. For further details I refer the reader 
to the excellent works of Purkinje, Raschkow and Frankel, 
and those of Retzius, J. Miller, and v. Linderer 

If an immature tooth of a child or mammal (the pig, for 
instance) be removed from its capsule and placed in dilute 
hydrochloric acid, the organic substance of the enamel which 
remains after the solution of the earthy matter, may be sepa- 
rated from the ivory entire. It has exactly the form and size 
of the enamel previous to the action of the acid. It is very 
soft, and breaks readily in the direction of the fibres of the 
enamel. Examined with a high magnifying power and subdued 
light, it is found to be composed, like the enamel itself, of 
closely-aggregated prisms, which may be insulated from one 
another, so that each one forms an independent structure. 
(See pl. III, fig. 3.) This organic substance, therefore, cannot 
be, as Raschkow and Retzius considered, a mere deposit from 
the moisture with which the enamel-fibres are at first sur- 
rounded, and thus a sort of cast of the enamel-fibres, but 
either the fibres must result from an ossification of these 
prisms, or the prisms must be hollow, and the imorganic 
substance deposited within them. When the enamel of the 


pig’s tooth is examined with a subdued light, the contour of — 
these organic prisms is found to be so dark in comparison with | 
their interior, that it can scarcely be regarded as the mere — 


shaded outline of a solid prism, but suggests the idea of a— 


cavity surrounded by a thin membrane. This distinction is, 


4 
5 
a 
\ 
i 
( 


P | 
1 


i 


ENAMEL OF THE TEETH. 99 


however, much less striking in human teeth, so that the ques- 
tion as to which of the two views is correct must remain un- 
decided. 

What, then, is the process of formation of these enamel- 
prisms? According to Purkinje and Raschkow, the crown of the 
growing tooth is surrounded externally by a peculiar membrane, 
the enamel-membrane, the inner surface of which is composed 
of short hexagonal fibres, which stand perpendicularly upon the 
membrane, and are directed towards the enamel, so that each 
fibre of the enamel-membrane corresponds to an enamel-fibre. 
On examining a portion of this membrane, particularly that 
part which les nearest to the root of the tooth, we readily 
recognize in it the characteristic nuclei, some of them being 
furnished with nucleoli. They le in a minutely granulous 
substance. This granulous aspect, however, is seen to be pro- 
duced, in many situations, by granulated cells which contain the 
nuclei. Hach nucleus is surrounded by a circular areola of 
small granules, and seems to lhe in a minutely granulated 
globule, which we know to be the rudimentary form of 
most elementary cells. Some of these cells are prolonged 
into very delicate fibres; they appear to be young cells of 
areolar tissue; most of them, however, are round. The 
fibres or prisms of the membrane, which have a direction 
from its inner surface towards the enamel-fibres, have 
assumed an hexagonal form, which Raschkow attributes to 
their close contact. They very closely resemble the columnar 
epithelium upon mucous membranes, only that they are 
prismatic in their entire length, that is, so far as they project 
out from the membrane to which they are attached. I am 
inclined therefore, to regard them as merely elongated cells. 
In the recent state they also contain a very distinct nucleus, 
which encloses its nucleolus. (See pl. III, fig. 4.) In the upper 
part of the enamel-membrane they lie quite close together ; 
but in the portion nearest to the root of the tooth, they 
diminish in number and stand insulated, so that at this 
part the structure of the membrane beneath them may 
also be recognized, and I suppose the round cells before 
mentioned to be the earlier condition of these prismatic 
cells. What, then, is the relation which these prismatic cells 
of the enamel-membrane bear to the prisms of the enamel? 


100 ENAMEL OF THE TEETH. 


Purkinje and Raschkow regarded each fibre of the enamel- 
membrane as an excretory organ, a little gland which secreted 
the enamel-fibre corresponding to it. With our altered views of 
the growth of unorganized ' tissues, however, this explanation, 
previously so plausible, loses much of its probability. Various 
other explanations might be offered in place of it, but I have 
not made suflicient observations to enable me to decide upon 
the correct one. Firstly, one might suppose the organic basis 
of the enamel-prisms to be cells, which are formed, and continue 
to grow independently upon the dental substance, having no 
other connexion with the prisms of the enamel-membrane than 
that the latter furnishes their cytoblastema. This explanation, 
however, would compel us to regard the remarkable accordance 
which exists between the prisms of the enamel-membrane and 
those of the enamel as an accidental circumstance. But we 
should be obliged to adopt such a view, if it could be proved 
that another peculiar substance intervened between the enamel- 
membrane and the enamel, and I have several times observed 
such an one on the molar teeth of swine. It is very soft and 
full of vesicles, having the appearance of a slag. I think 
Purkinje mentions it also, but I cannot find the precise passage 
at this moment. It lay between the enamel-membrane and 
the tooth, but I am not certain whether it was also present 
at those points where the formation of the enamel had already 
commenced, and whether, therefore, it actually interrupted the 
continuity of the enamel-membrane with the formed enamel. 
We might suppose, as a second explanation, that the enamel- 
prisms are uninterrupted continuations of the prisms of the 
enamel-membrane, which become filled towards one end with 
calcareous salts. This is a very improbable explanation, and 
the connexion between the two structures is of too loose a 
nature to warrant its adoption. A third, and as I am at pre- 
sent disposed to think the most probable, explanation is, that 
the prismatic cells of the enamel-membrane separate from it, 
and coalesce with the enamel already formed, while at the same 
time their cavities either become filled with calcareous salts, 
or they become ossified throughout their entire thickness, their 
cavity being previously filled with an organic substance. This 
explanation makes the formation of the enamel accord with 


' [The author appears to use this word as synonymous for “ non-vascular.”—TRANS. | 


(ie Fg » 


Peat met Se 


IVORY OF THE TEETH. 101 


the growth of the other unorganized tissues treated of in the 
previous class. If we suppose, for example, that the little 
cylinders (columnar epithelium) of the mucous membranes 
(which, according to Henle, are constantly being thrown off) 
could become ossified at the moment when they separated from 
the surface of the mucous membrane, we should obtain a cover- 
ing to the membrane, consisting of little calcareous cylinders, 
each of which, however, would still have its organic basis like 
the enamel-fibres. Beneath this covering would be other 
cylinders not as yet ossified, which, when they in like manner 
became calcified would add to its thickness, whilst new cylinders 
grew forth from the mucous membrane. The quantity of the 
organic basis is extremely small in the teeth of adults which 
haye been exposed for a considerable time to the action of the 
saliva, a circumstance which I suppose to be referrible to its 
undergoing a chemical solution in that fluid. 


b. The wory. 


This is known to consist of a structureless! substance, 
traversed by a great many minute canals. These canals 
(tubes) have for the most part a radiate course from the 
cavity of the tooth towards its external surface, and, accord- 
ing to Retzius, often give off branches as they proceed. 
Their peripheral terminations are extremely minute; they are 
thicker towards the dental cavity, and, when the pulp is removed, 
open freely into it. Miiller observed that the tubes projected 
beyond the intermediate substance from the fractured surface 
of thinly-ground laminz, and of lamellae which had been 
macerated in hydrochloric acid, and were surrounded therefore 
by a special membrane; Retzius also remarked the same 
upon a transverse section. Purkinje and Miller noticed that 
when teeth are placed im ink, the fluid penetrates into the 
tubes ; they must therefore be hollow. If any of them contain 
calcareous matter, it must be only the most minute ones. 
According to Retzius, many teeth present corpuscles which 
resemble those of bone, and like them send forth minute 
radiating canaliculi. 


' See note to page 39. 


102 IVORY OF THE TEETH. 


What relation then does the ivory bear to the cells? I 
must at once avow that I cannot give a positive reply to 
this question, and that I only communicate the following im- 
perfect investigation for the sake of presenting a connected 
view of my subject. The formation of the dental substance 
is described by Purkinje and Raschkow as follows : “ Primordio 
substantia dentalis e fibris multifariam curvatis convexis late- 
ribus sese contingentibus ibique inter se concrescentibus com- 
posita apparet. . . . . In ipso apice iste fibre equaliter quam- 
cunque regionem versus se diffundunt, attamen parietes laterales 
versus directiolongitudinalis preevalet, dum fibree sinuosis flexibus 
eequalique modo se invicem contingentes ibique ubi concave 
apparent lacunas inter se relinquentes, ab apice coronali radicem 
versus ubicunque procedunt. Non nisi extremi earum fines 
tune molles sunt ceterze autem partes brevissimo tempore in- 


durescunt. . . . . ‘Substantize dentalis formationis secundum 
crassitudinem processus pari modo ac primo ejus ortu cogi- 
tandus est. Postquam.... . fibrarum dentalium stratum 


depositum est, idem processus continuo ab externa regione 
internam versus progreditur, germinis dentalis parenchymate 
materiam suppeditante. .. .. Convexee fibrarum dentalium 
flexuree, que juxta latitudinis dimensionem crescunt, dum ab 
externa regione internam versus procedunt, sibi Invicem appo- 
site continuos canaliculos effingunt, qui ad substantiz dentalis 
peripheriam exorsi multis parvis anfractibus ad pulpam dentalem 
cavumque ipsius tendunt, ibique aperti finiuntur, novis ibi, 
quamdiu substantiz dentalis formatio durat, fibris dentalibus 
aggregandis inservientes.” (Raschkow, Meletemata circa Mam- 
malium dentium evolutionem. Vratislav. 1835, p. 6.) 

I must admit that I do not clearly understand some of this 
description, but if I rightly comprehend it, the dental substance 
originates from fibres which are formed in strata around the 
pulp (the latter supplymg the material for the purpose) ; 
that these fibres then coalesce, leaving, however, spaces be- 
tween them which are the dental tubes. Since, according 
to Miller, the tubes are furnished with special walls, we can 
no longer regard them as mere spaces between the fibres. 
His observation, however, does not affect the explanation of 
the formation of the firm substance. 


IVORY OF THE TEETH. 103 


If a tooth be removed from its capsule, and macerated 
for some days in slightly diluted hydrochloric acid, the dental 
substance, which on the first withdrawal of the calcareous salts 
possessed a cartilaginous consistence, becomes so very soft that 
it can only be removed from the acid in very small portions 
with the forceps. This pappy mass is found on examination 
to consist of fibres, which may here and there be insulated. 
(See pl. III, fig. 5.) These fibres are too thick to be the 
walls of the tubes; they form the entire substance. Nor 
can they well be an artificial product, the result of the acid 
penetrating into the tubes, and dissolving, in the first instance, 
the substance in immediate contact with them, so that the 
intercellular substance remained undissolved in the form of a 
fibre; they are too regular and smooth for that. It appears 
rather that the dental substance is composed of these fibres, 
which have become blended together, that they are therefore 
identical with those fibres, by the coalescence of which, accor- 
ding to Purkinje and Raschkow, the dental cartilage is formed, 
and that this coalescence is not so complete, but that it may 
be artificially dissolved. The fibres have the same course as 
the tubes in human teeth, but I could no longer perceive the 
tubes between them in this preparation; I could, however, 
recognize the fibres everywhere, save in the most external layers 
which lay immediately under the enamel, in which situation 
the mass was more completely broken down by the acid, and 
traversed by more minute fibres of a different kind, having 
the most confused and varied directions, and which I suppose 
to have been the remains of the dental tubes. 

We must therefore regard the dental substance as composed 
of fibres blended together, between which run tubes provided 
with special walls. The fibres and tubes are nearly per- 
pendicular to the dental cavity in human teeth. What con- 
nexion now is there between the fibres, or the tubes, and cells ? 
I should incline to the old opinion, that the dental substance 
is the ossified pulp. According to Purkinje and Raschkow, 
the pulp in the first instance consists of globules, of nearly 
uniform appearance, but has neither vessels nor nerves. At a 
subsequent period vessels appear in it, and afterwards nerves. 
Upon the surface of the pulp, the globules are more regularly 
arranged, and more extended in the longitudinal direction, 


104 IVORY OF THE TEETH. 


and are directed towards the outside perpendicularly, or at a 
slightly acute angle. These elongated globules are clearly 
cylindrical cells. In recent teeth, the characteristic nucleus 
with its nucleoli may be distinctly seen in them, and 
they very closely resemble the prisms of the enamel-membrane. 
(Pl. III, fig. 4.) The interior of the pulp also consists of 
round nucleated cells, between which the vessels and nerves 
pass. When the pulp of a young tooth is detached from its 
cavity, and the dental substance is examined (without further 
preparation, or after the earthy matter has been withdrawn), 
a stratum of the cylindrical cells of the pulp will be found 
to remain attached to its internal surface, at least to the 
lower part of it, where the newly-formed dental substance 
is yet thin and soft. These cells are of about the same 
size, and have the same course as the solid fibres of the 
dental substance ; and since, on the one hand, they clearly 
belong to the pulp, which follows from their accordance with 
the cylindrical cells that remain attached to the rest of its 
surface, and as, on the other hand, they are still more firmly 
connected with the dental substance than with the pulp, and 
remain affixed to the former, I suppose a transition to take 
place at that part, and the cylindrical cells of the pulp to be 
merely the earlier stage of the dental fibres, 1. e. that the cells 
become filled with organic substance, solid and ossified. In 
some instances, these little cylinders are not found upon 
the dental substance, but a quantity of cell-nuclei seen 
in their place ; these are very pale, and so intimately united 
with the dental substance, that they readily escape notice; when, 
however, attention is once attracted to them, it is impossible to 
mistake them, and they le side by side with extremely 
small interspaces. The facility with which the two struc- 
tures may be separated, has been adduced as an argument 
against the opinion that the dental substance is the ossified pulp, 
and I fully acknowledge the weight of the objection. But the 
following circumstances deprive it of at least some of its import- 
ance. Firstly, some portion of the pulp actually remains 
attached to the dental substance; again, in ribs which are half 
ossified, the cartilage may easily be separated from the ossified 
portion; and lastly, the separation must be effected with 
more facility in the tooth, in consequence of the greater 


7 . = “ 
ees 4 - 


IVORY OF THE TEETH. 105 


difference in consistence between the dental substance and the 
pulp. There are therefore, at least, reasons enough to war- 
rant our entering more particularly into the details of this 
opinion. The pulp accords with all the other tissues of 
the feetus, therefore with cartilage, in being composed of 
cells: the difference between its consistence and that of the 
cartilage of mammalia, depends on this, that the quantity of 
cytoblastema (to which the latter owes its hardness) is very small, 
for the cylindrical cells of the pulp le quite close together, 
at least such is the case on its surface. In this respect, the 
pulp bears a closer analogy to certain cartilages of animals 
lower in the scale, in which there is also only a small quantity 
of cytoblastema present, and the consistence of the cartilage is 
principally occasioned by thickening of the cell-walls. As I 
have not actually observed the- transition, 1 do not know 
whether the filling up of the cavities also takes place by thick- 
ening of the cell-walls, in this supposed conversion of the cells 
of the pulp into the dental fibres. If such be really the case, 
the cavities of the cells are in general so completely obliterated 
by it, that no cartilage-corpuscles remain. From the observa- 
tions of Retzius, however, it might be supposed that some of 
the celis retain their cavities, and even become transformed 
into stellated cells; for he saw true csseous corpuscles in the 
dental substance. When the uppermost stratum of the pulp 
consisting of cylindrical cells has become converted into dental 
substance by ossification, the round cells lymg immediately 
next beneath it in the parenchyma of the pulp, must first com- 
mence their transformation into cylindrical cells, the vessels of 
the stratum must become obliterated, and then this stratum 
ossified, and so on. 

What, then, are the dental tubes? Retzius compares them 
to the calecigerous canaliculi of bone which issue from the 
osseous corpuscles, and I was myself at first of that opinion ; 
for I regarded them as prolongations of cells, the bodies of 
which lay in the pulp. For, when the pulp is drawn out 
from the cavity of a pig’s tooth, and its margins examined, 
it will be seen that each of the cylindrical cells of the surface 
of the pulp becomes elongated into a short minute fibre towards 
the dental substance, and that these fibres are about as nume- 
rous as the tubes projecting upon the surface of the pulp. I 


106 OSSEOUS SUBSTANCE OF TEETH. 


thought formerly, that they became elongated to form the dental 
tubes, and that the intertubular substance was merely intercellu- 
lar substance between these prolongations. But I was compelled 
to relinquish that idea in consequence of there being no such 
appearance in the human tooth, and because the explanation 
led to difficulties with respect to the teeth of the pike, in 
which, according to Retzius, an immediate transition of the 
dental into the osseous substance takes place. If one of the 
largest teeth in the lower jaw of the pike be sawn off, deprived 
of its earthy matter by means of hydrochloric acid, and 
then divided into thin longitudinal sections, the dental sub- 
stance will be seen to form a hollow cone, which is filled with 
osseous substance. The dental substance is transparent,and con- 
sists of fibres which have a direction from the point towards the 
base of the cone. Canals traverse the osseous substance, 
resembling the Haversian medullary canals of ordinary bone, 
only they are not so regular. The dental tubes then are con- 
nected with these canals of the proper osseous substance, and 
may be distinctly seen issuing from them in a funnel-shaped 
form. The canaliculi soon ramify in the dental substance, 
and, as they run across the thickness of the dental cone, inter- 
lace with the dental fibres. According to this view, the 
dental tubes would correspond to the medullary or Haversian 
canaliculi of bone, and not to the calcigerous canaliculi pro- 
ceeding from the osseous corpuscles. It appears impossible, 
however, to be assured of the right explanation of all the struc- 
tural relations of the dental substance, until its development is 
examined in teeth differing widely from each other in con- 
struction. 


c. Osseous substance of the teeth. 


This requires no particular explanation, as it entirely ac- 
cords with the ordinary osseous substance. 


Having examined in detail the tissues comprehended in this 
class, and compared them one with another, we have yet to 
consider the entire class in relation to those which have been 
previously discussed, and to observe how much our knowledge 
of the transformations which the cells are capable of under- 
going, has been advanced by the study of it. 


anit. 


= es Se ee eee eee ee ey 


ee ee ae ee 


COMPARATIVE RETROSPECT. 107 


It is easy to see which elements of the tissues of this and 
the preceding class correspond. There the whole tissue 
consisted of cells, closely crowded together, and the in- 
tercellular substance was almost nil. Here we find the like 
arrangement only in the lowest stage of development of the 
most simple cartilages. In such as are more highly developed, 
those of all the mammalia for example, the cells lie surrounded 
by a larger quantity of intercellular substance, which forms 
the proper cartilaginous substance ; but the cell-walls contri- 
bute only very slightly, or not at all, to its formation. The 
proper firm substance of these higher cartilages, therefore, has 
its analogy in the former class, only in the minimum of cyto- 
blastema by which the cells are connected, while, on the other 
hand, it corresponds with that which, in the first class, was 
the fluid, wherein the isolated cells were formed. The carti- 
lage-cells in this class, however, correspond precisely to the 
epithelium-cells, the feather-cells, &. &c., in the preceding 
one, and the blood-corpuscles, mucus-corpuscles, &c. in the 
first class. 

We have not found any new changes in the form of the 
cells in this class. Most of them were angular, somewhat 
approaching the circular form; and stellated cells, so far at 
least as we may be permitted to regard the osseous corpuscles 
as such, were also frequently met with. (See pp. 29, 30.) 
Some cells, which were remarkably elongated, were observed 
near to the surface of several cartilages, in which situation 
they are known as greatly elongated cartilage-corpuscles ; 
still, however, this appearance is never presented by the cells 
of this class in so remarkable a degree as it is by those 
of the crystallme lens in the previous one. The fibro- 
cartilages, on the other hand, form the immediate transition 
from this to the following class, for in them a bundle of fibres 
seems to be formed out of each cartilage-corpuscle, a process 
which we shall consider more minutely when treating of cel- 
lular (areolar) tissue in the next class. 

We have observed the formation of cells around the pre- 
viously-existing nucleus, and their progressive growth, going 
on in this class in a similar manner to that exhibited in the 
preceding, and the true cartilage-cells were also seen to form 
around a cytoblast which. lay external to the cells already 


108 COMPARATIVE RETROSPECT. 


developed. On the other hand, a formation of cells also takes 
place within the true cartilage-cells, but it is probable that 
they have a different signification from those within which they 
are generated. A deviation from the previous class seemed to 
occur with respect to the spot at which the young cells are 
formed, in relation to the entire tissue. In the former class, 
so far as we could perceive, they were formed at that part 
only where the tissue was in immediate contact with the 
organized substance. The formation of the new cells im 
cartilage, it is true, did not take place throughout the entire 
thickness of the tissue, but (so long at least as the cartilage 
itself is not furnished with vessels) only near the sur- 
face, and therefore, at the spot where it was in contact with 
the organized substance; still, however, it not only took 
place at that point of contact, but went on also between 
the cells most recently formed, as if cartilage had a greater 
capacity of imbibition, so that the cytoblastema penetrating 
from the blood-vessels into the parenchyma arrived at the 
deeper seated portions of the tissue more speedily ; and, there- 
fore, retained its fresh plastic force even in that situation ; 
or, as if the cartilage itself possessed a higher vitality, and, 
therefore, the cytoblastema retained its productive power for 
a longer period, although penetrating quite as slowly as in 
the previous class. 

Although the modifications in the form of the cells of this 
class vary but slightly from those of the preceding one, yet we 
see two striking changes in the cells and their cytoblastema, 
namely, the coalescence of the cell-walls and ossification. The 
thickening and transformation of the cell-walls were very dis- 
tinct in the last class, for example, in feathers. Here a still 
more strongly-marked thickening of the cell-walls takes place 
in several cartilage-cells. The external contours of the walls, 
however, gradually disappear in such instances, and a coales- 
cence takes place to such an extent as to leave merely the 
cell-cavities perceptible, lying in an homogeneous substance. 
The blending of the cell-walls takes place either between the 
walls of neighbouring cells, in instances where they are in 
immediate contact, or, with the intercellular substance, when 
the cells are surrounded by it. Further investigations are re- 
quired in order to decide the question as to whether this 


i te ee et a te a a i ll 


eS oa. oe 


EE ried tile te ae 


COMPARATIVE RETROSPECT. 109 


blending be so complete that it cannot in any way be dissolved, 
the simple fact being, that the cell-walls are no longer dis- 
cernible with the microscope. I shall not bring forward the 
splitting of -the dental fibres as examples here, nor indeed 
make any reference to the teeth in this retrospect, their ex- 
planation being as yet too problematical. It has, however, 
been already mentioned as a doubtful point, whether a coales- 
cence of the walls actually takes place in all cartilage-cells, 
for instance, in those of the higher animals. 

Ossification appears to occur especially, perhaps exclusively, 
in those cartilages which have a greater quantity of intercel- 
lular substance. It consists probably in a chemical union 
between the calcareous salts and the firm portion of the car- 
tilage substance. In the first commencement of the process 
the cartilage frequently acquires a granulous appearance, which 
subsequently disappears, the entire substance meanwhile be- 
coming gradually dark. At the same time the cartilage-cells 
undergo a transformation into the osseous corpuscles, a proéess 
which must probably be explained as analogous to the for- 
mation of the stellated pigment-cells. There is reason to 
suppose that the osseous corpuscles and the canaliculi which 
issue from them, also become filled with earthy salts by the 
calcifying process. 

The class of cells now under consideration has yet another 
point of especial interest for us, since it is the first in which 
organized structures, that is, structures provided with vessels, 
occur. The accordance between the elementary cells of 
‘unorganized animal tissues and vegetable cells might be con- 
ceded, without granting a connexion between the organized 
tissues (which are especially characteristic of animals) and 
the structure of vegetables. A distinction had always been 
drawn between the growth of the organized and that of the 
unorganized structures; and much had already been said 
im a general way about a vegetative growth of the non-vascular 
structures, the crystalline lens for instance, though the analogy 
which existed between their elementary particles was not 
proved. Cartilage, then, is the first structure which teaches us 
that a tissue, which, at a later period at least, contains vessels, 
is composed of cells, perfectly according, in their development, 
with those of plants; and, therefore, that a similar formative 


110 FIBRE-CELLS, ETC. 


principle is the basis both of the organized and unorganized 
tissues. We shall have further evidence of this presented to 
us in the following classes, which comprise the rest of the 
tissues,—those, indeed, which are most perfectly organized, 
and the most important to the animal organism. In them 
we shall also find that the formation of cells is the general 
principle of development, and that their elementary particles 
are derived from cells, although at the first glance one would 
scarcely imagine that any connexion could exist between them 
and cells. 


CLASS IV. 


Fibre-celis, or tissues, which originate from cells that become 
elongated into bundles of fibres. 


Mere fibres are all that can be detected as the elementary 
components of the tissues of this class when they are examined 
in the mature animal. But when we investigate the mode in 
which they are generated, we see that the fibres are formed 
only as prolongations of cells, which, in most instances, are 
elongated in two opposite directions, sometimes terminating 
at once in a fasciculus of fibres, at other times in a single fibre, 
which afterwards splits into several finer ones. This con- 
stitutes the characteristic feature of the class. We are already 
acquainted with the type of the prolongation of cells into fibres 
in the pigment-ramifications, osseous corpuscles, &c. The 
cells next to be considered differ from them in the followimg 
particulars: the fibres originating from any one cell generally 
lie together in a fasciculus, and in these prolongations of the 
cells, it is principally the wall which is most strongly developed, 
whilst, in the former instances, the cells though extended into 
fibres, were chiefly rendered conspicuous by their cavities. 
This class comprises the Cellular (areolar), Fibrous, and Elastic 
tissues. 


1. Cellular (areolar) Tissue. This tissue is known to. be 
composed of extremely minute, tough, smooth fibres, having 
a pale outline, and usually a serpentine course; they 
may be seen in their natural state in the mesentery with- 


| 
| 


AREOLAR TISSUE. 111 


out any dissection. Most areolar tissue may be distended by 
forcing air into it, and then innumerable cellular spaces are 
seen communicating with each other in it; it is not known 
whether these are produced artificially, or whether they existed 
previously. Areolar tissue also frequently contains fat-vesicles, 
which, accordiig to Gurlt, are surrounded by a thin and 
transparent, but not fibrous, pellicle, often have an hexagonal 
form, and in that respect resemble vegetable tissue. (Gurlt’s 
Physiologie der Haussaiugethiere, p. 19.) In order to become 
acquainted with the relation which these constituent parts of 
areolar tissue bear to the elementary cells, we must refer to 
the formation of the tissue in the foetus. 

If we examine some areolar tissue from the neck, or from 
the bottom of the orbit of a foetal pig measuring three inches 
and a half in length, we shall find it to be a gelatinous substance, 
somewhat more consistent than the vitreous humour of the eye, 
and, in its earliest state, quite as transparent ; as development 
proceeds, however, it becomes more of a whitish colour, and 
loses its gelatinous quality. When examined with the micro- 
scope, small corpuscles of various kinds are seen in greater 
or less numbers; they are not, however, sufficiently numerous 
in a foetus of the size specified to form the entire gelatinous 
substance, but must necessarily be situated in a transparent, 
structureless,' primordial substance of a gelatinous nature, which 
we will for the present call cytoblastema. The whiter this 
substance appears to the unaided eye, the greater is the 
number of corpuscles contained in it; their quantity, there- 
fore, is continually increasing during development, while 
that of the cytoblastema constantly diminishes. As in con- 
sequence of its transparency, the cytoblastema cannot be 
seen, but is only inferred to exist from the circumstance 
that the corpuscles, which are visible under the microscope, 
could not, at the period when they are but few, form the 
entire jelly, and that when moved, it is plainly seen that 
they are held together by some invisible medium, so it is no 
longer possible to convince ourselves of its existence, when the 
corpuscles are very numerous. It is probable, however, that it 
remains between the fibres of the areolar tissue throughout life. 


' Vide note at page 39. 


co 


112 AREOLAR TISSUE. 


This cytoblastema is present in the greatest quantity, and 
therefore most distinctly demonstrable in the jelly which lies 
between the chorion and amnion in the foetus of the pig at a 
somewhat more advanced period, end where it may be rendered 
very clearly perceptible on the margin of the preparation by 
colouring it with iodine. It is quite as evident in the cellular 
tissue of the young tadpole. An indistinct fibrous appearance 
is sometimes given to it by drawing it asunder; but a fibrous 
structure must not be inferred from that fact simply, since all 
tenacious matter assumes that appearance under similar cir- 
cumstances. Since the number of the corpuscles in the cyto- 
blastema continually increases as development proceeds, it 
would appear that the cytoblastema must be regarded as the 
primary formation, so that we may suppose some of it to be 
first present, and then the corpuscles originate in it; at the 
same time, however, new cytoblastema is formed, in which new 
corpuscles are in like manner generated, whilst the formation 
of those in the previously-existng cytoblastema proceeds 
simultaneously. 

Three kinds of these corpuscles may be distinguished in the 
mammalian embryo; one, which is developed at an earlier 
period than the rest, and is found in all the areolar tissue 
throughout the foetus, and two others, which are formed 
subsequently, and, as it would seem, do not occur in the areolar 
tissue of some parts. We shall, therefore, designate the first 
(which is the only essential kind) proper corpuscles of areolar 
tissue, or—in accordance with the signification which will 
shortly be determined for them—fibre-cells of areolar tissue ; 
the second kind are fat-cells ; the third form round cells of 
areolar tissue, the precise signification of which I have not yet 
been able to make out. 


a. Proper corpuscles of areolar tissue, or fibre-cells of areolar 
tissue. The areolar tissue is not found in the same stage of 
development in every part of the same foetus. When some of 
the tissue that has reached about its middle stage of develop- 
ment is removed from the neck of a pig’s foetus, measuring 
from four to seven inches in length, and examined with the 
microscope, a quantity of corpuscles of various forms are ob- 
served in it. The majority of them, however, appear as they 


a ee 


* 


AREOLAR TISSUE. 113 


are represented in pl. ITI, fig. 6, a, being spindle-shaped or 
longish corpuscles, which are thickest in the middle and gra- 
dually elongated at both extremities into minute fibres. They 
may therefore be described as consisting of a thicker portion, 
or body, and fibres, which proceed from it. 

The body is either round or slightly compressed upon the 
sides. The surface is covered with very minute granules. 
Within the thickest portion of it lies ‘another small corpuscle 
of a circular or generally oval form, and which again encloses 
one or two small dark points, and accords entirely with the 
common cell-nucleus. It is therefore probable, that the entire 
corpuscle is a cell containing a nucleus. The nuclei have not 
a similar size in all the cells; there is a much more striking 
variation, however, in the relative size of the cells and the 
nuclei. In the largest cells, such as a, fig. 6, the body is 
almost as thick again as the nucleus, and it may be observed 
that the nucleus does not lie in the centre, but upon the wall 
of the cell. In most instances, however, the cells are rela- 
tively smaller, scarcely larger indeed than the nucleus ; 
insomuch, that the fibres often appear to proceed immediately 
from the nucleus, as at 6 in the figure: the cell in 
that imstance encompasses the nucleus quite closely. Cells 
frequently become separated during the process of preparation 
for the microscope, and float about singly in the water, with a 
portion of the fibres issuing from them. By causing them to 
roll, when so detached, it may be satisfactorily seen that many 
of them are somewhat flattened laterally, and that the nucleus 
is attached to the inside of the cell-wall. The larger cells, 
under such circumstances, appear as though the granulous 
aspect were produced by the external wall only, therefore by 
the cell-membrane, the interior being filled with a clear fluid. 

The cells pass by a gradual process of acumination into the 
fibres, it being quite impossible to discern any defined boun- 
dary between them. ‘The fibres are pale, minutely granulous 
like the cells, and frequently give off branches. Their course 
is usually straight. It is difficult to find out how they terminate ; 
but they are generally lost in a bundle of extremely minute 
fibres. 

The above-described corpuscles, then, are the fibre-cells of 
areolar tissue in the middle stage of their development, a con- 

8 


114 AREOLAR TISSUE. 


dition in which they immediately attract attention im the 
investigation of that tissue in the foetus. We shall in the next 
place consider the earlier, and then the subsequent stages of 
their development. In addition to the corpuscles before men- 
tioned, others may be seen in very young areolar tissue, which 
are not elongated into fibres, but are more or less round. They 
are granulous and contain a nucleus with nucleoli, and as they 
present all the stages of transition up to those cells which are 
prolonged into fibres, we must regard them as being the un- 
developed fibre-cells. Various forms of them are delineated 
in pl. III, fig. 6. I will not assert that all round cells in 
foetal areolar tissue are young fibre-cells; for we _ shall 
presently become acquainted with some which are not. It 
is only after the commencement of the process of acumina- 
tion that the young fibre-cells can be distinguished from these; 
in the earliest state, when they are as yet quite round, almost 
all cells are alike. It is difficult to determine positively 
whether or not these cells are formed around a previously 
existing nucleus ; probably, however, such is the case, as there 
are no cells to be seen without nuclei, although there are many 
nuclei observed without investing cells. 

The following, then, are the results of our investigation into 
the progress of development of areolar tissue, in so far as 
we have as yet pursued it. In the first place, small round 
cells are formed (probably around a previously existing 
nucleus), in the structureless jelly-like cytoblastema of the 
tissue. The cells, furnished with the characteristic nucleus, 
become acuminated in two opposite directions, and these acumi- 
nations elongate into fibres, that sometimes give off branches, 
and at length split into fasciculi of extremely minute fibres, 
which, in the early stage, cannot be distinctly perceived to be 
insulated. As development proceeds, the splitting of the two prin- 
cipal fibres, issuing from the body of the cell into a bundle of 
more minute fibres, continually advances nearer towards the cell, 
so that, at a later period, a fasciculus of fibres issues immediately 
from the body of the cell (see pl. III, fig. 7.) At a subsequent 
period, this process of splitting reaches as far as the nucleus, 
and at length goes quite through the body of the cell, 
and the nucleus then lies merely upon a fasciculus of 
fibres. At the same time the fibres in the progress of de- 


AREOLAR TISSUE. 115 


velopment are rendered smooth, become distinctly and indivi- 
dually discernible, and assume their waving course; in short, 
they acquire the appearance of the ordinary fibres of areolar 
tissue. (See the figure.) As the process of splitting advances 
from both sides towards the nucleus, the fibres in its neigh- 
bourhood are those which are longest united together, and 
that part of the cell is the last to undergo division. The 
nucleus remains for a time lying upon the fasciculus of 
fibres ; and when it is at last absorbed, we have a bundle of 
fibres in the place of the original cell. The figure repre- 
sents a nucleated cell, which is elongated at the upper end 
into the characteristic fibres of areolar tissue, each one being 
individually perceptible; the upper part of the body of this 
cell has also begun to split into fibres. With regard to 
the elongation downwards, it is not possible to distinguish 
whether there are separate fibres yet formed, and collected into 
a cord, or whether it is still merely a simple prolongation of 
the cell. 

It now becomes a question how the elongation of the 
cells into fibres, and their division, and at a later period 
the splitting of the body of the cell also into more minute 
fibres, can be conceived to take place. We have already 
observed a prolongation of the cells into fibres in several in- 
stances, and have traced it minutely in the stellated pigment- 
cells. The only difference between them and the fibre-cells of 
areolar tissue is, that in the latter, the elongation generally 
takes place in two opposite directions only, a circumstance 
which also frequently occurs with pigment-cells ; whilst, on the 
other hand, the cells of areolar tissue also frequently become 
elongated into fibres on several sides ; see, for example, pl. III, 
fig. 8. There is often a striking resemblance in form between 
some of the cells of areolar tissue and those of pigment ; com- 
pare, for instance, pl. III, fig. 6 a, with pl. I, fig. 8e. Analogy 
would lead us to regard those fibres as hollow; but since the 
cell-contents are not so characteristic in them as they are in 
the pigment-cells, a cavity might really exist, but not fall 
under observation, in consequence of the minuteness of the 
fibre ; the appearance of the fibres, therefore, proves nothing, 
either in favour of or against their hollowness. Since, however, 
we are already acquainted-with many extremely minute hollow 


116 AREOLAR TISSUE. 


prolongations of cells, and as the transformation of the cells 
into fibres in the areolar tissue, takes place by a gradual 
acumination, it seems to me, for the present, more probable 
that they are hollow rather than solid. If we imagine the 
formation of the fibres from a cell to take place by the 
cell-wall growing more vigorously at two opposite limited 
spots than it does at any other part, we can then conceive 
that the division of these main fibres into branches, and their 
prolongation into fibrils, may be effected by the same process. 
The question as to the hollowness or solidity of these fibrils, is 
still less capable of being settled by observation than that 
with respect to the larger fibres. Analogy is in favour of 
their being hollow, and the mimuteness of an object forms 
no limit to nature’s operations. 

The splitting into fibres, which, as we have seen, pursues a 
retrograde course from the branches towards the main fibres, 
and thence towards the body of the cell, might be illus- 
trated in the followmg manner :—suppose that part of a_ 
glove which corresponds to the hand to be the body of a cell, 
and the fingers to be a fasciculus of fibrils. If the membrane 
situate in the angle between two fingers grow in the 
direction of the hand, the glove will at length be split into 
five portions. But a difficulty arises with respect to the 
fibre-cells of areolar tissue, which is, that the division imto 
fibres advances from two opposite sides towards the body of the 
cell, and, therefore, the fibres of one side must ultimately cor- 
respond with those of the other. This, however, admits of no 
further explanation than the healing of the corresponding pri- 
mitive fibres in the reproduction of nerves does. Meanwhile 
the above are only attempts to convey a clear idea of the 
results of my imvestigations, modes of representation which are 
susceptible of various modifications, provided they be not made 
to contradict the observations ; the latter may be briefly summed 
up as follows :—cells, furnished with the characteristic nucleus, — 
are present in the first instance, which become elongated on 
two opposite sides, more rarely on several sides, into fibres, and 
these are prolonged into more minute fibres. Ata later period 
the principal fibres, and then also the bodies of the cells are 
split into fibres, so that a small fasciculus of fibrils, with a— 
nucleus fixed upon it, remains in the place of the original single 


AREOLAR TISSUE. Wig 


cell. Last of all, the nucleus also disappears, and fibrils alone 
remain. All these transformations proceed in a homogeneous 
eytoblastema, which probably also continues to exist between 
the fibres of areolar tissue in the adult. 


b. Adipose cells. In the later periods of foetal existence, 
adipose cells occur in many situations in addition to the fibre- 
cells before described. They are usually first seen in small 
groups between the fibre-cells. They are round cells of very 
various sizes, which are generally completely filled by a single 
fat-globule. The cell-membrane which closely encompasses 
the contents, is most minutely granulous, or, according to 
Gurlt, homogeneous. It is in most instances very thin, being 
about half the thickness of a blood-corpuscle, but sometimes it 
is much thicker, and in the subcutaneous areolar tissue of the 
thigh of a rickety child, at the age of twelve months (probably 
in connexion with the disease), was almost as thick as the 
breadth of a human blood-corpuscle. In the early stage, this 
cell-membrane encloses a very distinct nucleus of a round or 
oval form, which is sometimes flattened. When the former is 
thin, the nucleus presents itself externally as a little promi- 
nence upon the round fat-globule, which is closely encom- 
passed by the cell-membrane ; but when thick, the nucleus lies 
embedded in it. It contains one or two nucleoli. It is not 
uncommon for adipose cells to contain a number of small 
globules instead of one fat-globule, in such instances, one of 
them is generally remarkable for its larger size. The adipose 
cells are best seen in the fat found in the cranial cavity of a 
young carp, before it has attained the length of six inches. 
(See pl. III. fig. 10.) They there lie in so soft a substance, 
that they may be insulated without any difficulty, and float 
singly in the water in which they are examined. Some are so 
large as to be visible even with the unaided eye. When 
examined under the microscope with a magnifying power 
of 450, the cell-membrane is readily recognized, it is very 
thin, and closely encompasses the contents. It rises into 
a little prominence on one side, within lies a proportion- 
ately large, and very beautiful cell-nucleus, which is oval, 
but not flattened, and contains one or two very distinct 
nucleoli. Some of these fat-cells have two such nuclei, which 


118 AREOLAR TISSUE. 


have precisely similar relations to the cell, and both elevate 
the cell-membrane into a prominence at the points where 
they are attached. When one of these cells is pressed 
under the compressorium, the cell-membrane is at first 
remarkably expanded, and then tears to a very limited 
extent, allowing the fat to flow out. When the pressure is 
discontinued, it contracts again strongly. It has a minutely 
granulous aspect, is soft and very elastic, but not fibrous. 

In close apposition, the cells become flattened against one 
another into polyhedral shapes, and, as Gurlt remarks, they 
then resemble vegetable cells in their appearance. We, how- 
ever, may go further, and regard them as corresponding in 
signification also. In them the fat forms the cell-contents, as 
the pigment does in its cells, and the ethereal oil, &c. in those 
of plants. In its physiological signification of nutritive deposit 
it has more analogy with starch than with any other substance. 
I know not whether the nucleus is the part first formed in 
these cells, or not. Nuclei without any mvesting cells are 
found in the cranial cavity of the carp, lying with the adipose 
cells in the surrounding cytoblastema; these, however, may 
be nuclei of fibre-cells of areolar tissue. Sooner or later the 
nuclei become absorbed. They were still quite distinct in the 
adipose cells of the subcutaneous areolar tissue in the thigh 

of the before-mentioned rickety child twelve months old, 
whilst I could not detect any in the neck of a foetus at the 
seventh month. The absorption of the nucleus proceeds in one 
of two ways; either its external contour becomes gradually 
indistinct, some granulous substance merely being left in its 
place, which substance also disappears at a later period, or 


small fat-globules are formed both within the nucleus itself, 


and in its immediate proximity, which go on increasing in 
size, whilst the nucleus gradually disappears. The cell-mem- 
brane probably remains, even in the mature condition of the 
tissue, and Gurlt has made the very interesting observation, 
that im emaciated persons, the ordinary adipose cells are filled 
with serum, 


ce. The third kind of cells which occur in the areolar tissue 
of the foetus are round, for the most part extremely pale and 
transparent (pl. III, fig. 9.) They vary very much in size, 


—_ = = 


AREOLAR TISSUE. 119 


most of them being much larger than the fibre-cells, and some 
as large as the largest adipose cells. They can very rarely 
be seen without the aid of the most favorable light, but when, 
under such circumstances, the observer has once detected one 
of them, and become familiar with the degree of its trans- 
parency, they may be recognized in great numbers. They 
have a distinct nucleus attached to the internal surface of their 
wall, contaiming one or two nucleoli. The nucleus always 
attracts attention first ; the cell surrounding it is either quite 
transparent, and void of granules, or has granulous contents, 
and this granulous deposit is first formed in the neighbourhood 
of the nucleus, the remaining portion of the contents being 
still transparent. (See the figure.) Gradually, the entire con- 
tents appear to become granulous. These cells are distinguished 
from the fibre-cells of areolar tissue by the circumstance of 
their becoming much larger than the latter, and their not 
being elongated into fibres, and from the adipose cells, in that 
they do not contain fat. I have found them in areolar tissue 
taken from the bottom of the orbit, and from the neck of a 
foetal pig, but do not know whether they occur in the areolar 
tissue of all parts of the body; nor can I determine their sig- 
nification. They might be regarded as cellular spaces which 
had been produced by the distension of the areolar tissue with 
air. In such case, they must communicate with one another 
in the course of their further development. But this appears 
to me to be somewhat improbable ; and those spaces may be 
merely artificial productions. I should rather regard the cells in 
question as a modification of the adipose cells. For since, ac- 
cording to Gurlt, the ordinary adipose cells in the adult may 
contain mere watery fluid, one may also conceive the cells 
destined to the formation of fat becoming completely developed, 
without that formation actually taking place within them. 
There are, indeed, adipose cells which contain fat even in the 
earliest stage of their development, but that is no reason 
why the formation should not take place at a much later period 
in other cells. The granulous deposit in many of them might 
be regarded as the transitional step to the formation of fat. 
The cellular tissue of the foetus differs in its chemical con- 
stitution from that of the adult, since we cannot obtain any 


120 AREOLAR TISSUE. 


gelatine from it by boiling, none at least which has the pro- 
perty of gelatinizing. The integument was removed from a 
pig’s foetus measuring four inches in length, cut up into pieces, 
and steeped in distilled water for a day. It was then boiled 
for twenty-four hours. The last process caused it to crumble 
into small particles that clouded the fluid, in which also some 
large lamellz of epidermis floated. When examined with the 
microscope the epidermis exhibited the same structure as it 
did previous to being boiled ; the nuclei in the separate cells 
were also distinct. A quantity of fibre-cells floated in the fluid 
in the same state as when they, in their recent condition, com- 
posed the entire cutis, that is to say, longish corpuscles extended 
at both extremities into somewhat long fibres. The cell-nucleus 
could still be distinctly recognized in some of them. Thus 
the process of boiling, which had not produced any effect upon 
the fibre-cells or the fibres issuing from them, had dissolved 
the connecting cytoblastema, by which they had been held 
together in the recent state. The fluid was then filtered. 
Acetic acid caused a precipitate which was not dispersed by an 
excess of acid. <A solution of alum produced a much more 
copious precipitate, which, in like manner, was not redissolved 
by an excess of alum, or at least not completely. Tincture 
of gall-nuts caused a thick clouding, spirits of wine only a 
slight-one | Hydrochloric acid clouded the fluid, and an 
excess of acid did not render it clear again. These reactions 
accord with what Giiterbock has called pyine, save that the 
clouding produced in the latter by hydrochloric acid, was 
redissolved by an excess of the acid. <A portion of the filtered 
fluid was evaporated almost to dryness, but even after twenty- 
four hours, there was no trace of the formation of a jelly 
observable. In order to separate the component particles of 
this, in all probability, still very heterogeneous fluid, in some 
degree from one another, some pure alcohol was added to that 
portion of it which had been so long evaporated, whereby a 
very copious precipitate was produced. This was separated by 
filtration and washing, first with pure alcohol, and afterwards 
with spirits of wine of 80 per cent. strength, then dried, and 
again dissolved in boiling water. Acetic acid and alum caused 
precipitates in this solution, which were not again dissolved 


AREOLAR TISSUE. 121 


by the addition of either of those substances in excess. With 
respect to hydrochloric acid, the result was the same as before 
described. 

It cannot appear at all surprising that the areolar tissue of 
the foetus differs from that of the adult, when it is known that 
many cell-membranes undergo a change in their chemical 
constitution at different stages of their development, and that 
the growth of the cells is not a mere mechanical expansion. 

Previous to quitting the subject of areolar tissue, we must 
consider some other processes, by means of which a new for- 
mation of it takes place in the adult. If (as I have already 
laid down as an axiom in my first essays, Froriep’s Notizen, 
1838, Nos. 91, 103, and 112) the formation of cells be really the 
principle of development of all organic structures, it must apply 
no less to pathological than to physiological processes; and 
that it really does so, is proved by the investigations of Henle 
with reference to the new products resulting from inflammation, 
namely, exudation, suppuration, and granulation; the results of 
his observations are communicated in Hufeland’s Journal, 
vol. Ixxxvi, No. 5. 

Vogel pronounced the pus-corpuscles to be epithelium, in 
consequence of their resemblance to epithelial cells, and there 
was much of probability in the statement, so long as it ap- 
peared that every diversity in the physiological signification 
of an elementary structure was based upon a recognizable 
diversity of formation. But this conclusion lost its importance, 
when I brought forward the formation of cells as the common 
principle of development of elementary structures, which were 
perfectly distinct in a physiological sense, and at the same time 
showed the most opposite tissues to be developed from cells, 
which, in the first instance, perfectly resemble each other, and 
present no distinction either in appearance or in the signi- 
fication of their individual parts. Henle, however, proved a 
positive difference between the epithelial cells and pus-cor- 
puscles, for he found that the nuclei of the youngest epithelial 
cells were not broken down by the action of acetic acid like 
those of the pus-corpuscles. The latter must, therefore, be 
regarded as peculiar cells, which are developed in the serum 
of pus in the same manner that all other cells originate in 
their cytoblastema; the only difference being that in this 


122 AREOLAR TISSUE. 


case the cytoblastema is fluid. Beneath the pus, in healing 
wounds, lie the granulations, composed of a firm cytoblastema, 
in which lie a quantity of cells. Henle thus describes the 
microscopical structure of granulations: ‘“ The most superficial 
part presents cells, which resemble the pus-granules, except that 
their nuclei are not broken down by acetic acid. In the 
deeper strata, the nuclei are very distinct, and the envelopes 
are polygonal, in consequence of mutual pressure. Wood has 
already drawn attention to their resemblance to epithelial 
cells. Deeper still the envelopes of the cells are found passing 
through all the gradual transitions of the fibres of areolar 
tissue, just as in the immature areolar tissue of the embryo. 
The first rudiments of these fibres are the longish nucleated 
corpuscles, which Giiterbock observed, and compared to the 
cylindrical epithelium. Hence it follows, that the formation 
of new cells proceeds upon the surface of the granulations, 
and that the transformation of the latter into cellular tissue 
(cicatrix-material, narbensubstanz) proceeds successively from 
the bottom of the wound towards the surface.” As no gelatine 
can be obtained from the granulations by boiling, Guterbock 
thought that those fibres in the granulations and exudations 
which resemble the areolar tissue ought not to be regarded 
as the actual fibres of that tissue, but as merely those of fibrin. 
But, as we have seen above, the entire areolar tissue of the foetus 
also does not afford any gelatinizing gelatine ; and since Henle 
observed a similar course of development in these fibres to 
that which I had pointed out in the areolar tissue of the 
foetus, we must regard them as the young fibres of that 
tissue (although they may differ from the mature tissue in 
their chemical qualities), and the granulations as nothing more 
than a primitive formation of areolar tissue. 

A formation of areolar tissue similar to that in the foetus 
takes place also in exudations resulting from inflammation. 
R. Froriep (Klin. Kupfertafeln, llte Lief. Weimar, 1837, 
Th. lxi) had already observed that irregular granules, some of 
which seemed to be extended on one or both sides into thin 
fibres, existed in the exudation of pericarditis, in addition to 
the fibres resembling areolar tissue. ‘These elongated gra- 
nules of fibrin,” he continues, “ seem to be the commencements 
of the formation of the new mass of tissue, that is, the rudi- 


FIBROUS TISSUE. 123 


ments of the newly-forming cylindrical fibres of the areolar 
tissue composing the proper false membranes, or substance of 
the cicatrix.” Thus Froriep had already observed the gene- 
ration of fibres, resembling those of areolar tissue, by the elon- 
gation of corpuscles ; what he here calls fibrine-globules, are, 
no doubt, the nucleated fibre-cells becoming elongated into 
fibres. Henle examined the exudation by which wounds 
that heal by the first intention are closed, and found, that, 
in this also, cells are formed which undergo transformation 
into fibres of areolar tissue by an elongation of their envelope, 
just as in the foetus. He thence concludes, that the formation 
of exudations and granulations are essentially similar processes. 
The exudation-globules (exsudatkugeln) discovered by Valentin, 
and described also by Gluge, which, according to the former, 
occur in every form of exudation, are, he says, allied to pus- 
corpuscles ; and Henle also found that their nuclei are like- 
wise broken down by the action of acetic acid. 

Suppuration, therefore, differs from exudation and granula- 
tion only in this circumstance, that a more fluid cytoblastema 
is formed, in which fewer perfect cells are developed. It 
represents an intermediate stage between the formation of the 
firm tissues and the true function of secretion ; between which 
two processes again no essential difference exists. 


2. Fibrous Tissue. 'The fibres of tendons and those of 
areolar tissue, differmg but little from each other, and it 
being impossible to define precisely the respective limits of 
the two structures in the perfectly developed condition, we 
accordingly find that they agree in their mode of origin. 
Cells, resembling the fibre-cells of areolar tissue, are found 
in the tendons of the foetus at a very early age. They 
are arranged with their long axis corresponding to that 
of the tendon, and are prolonged in two opposite directions 
into fibres, which again subdivide into more minute ones. 
(See plate III, fig. 11.) These cells split imto fibres pre- 
cisely in the same way as those of areolar tissue; they have a 
nucleus similar to theirs in shape, which remains for a period, 
but is at last absorbed, leaving nothing but the fasciculus of 
fibres persistent. All these processes, however, take place 
much earlier in fibrous tissue than in the areolar, so that, 


124 ELASTIC TISSUE. 


unless the tissue is investigated in a very young foetus, we can 
only detect cell-nuclei intermixed with fibres, or nuclei, in 
whose immediate proximity a small fasciculus of fibres arises 
on both sides. At an early stage of development the tendons 
have a gray appearance, not having assumed the white colour 
of the adult tissue. This fact is probably connected with 
a chemical difference existing between the young and perfectly 
developed fibrous tissue, as in areolar tissue. The quantity of 
the cytoblastema in which these cells are formed, and by which 
the fibres and tendons, when perfected, are probably connected 
together, must be extremely small, and cannot in any way be 
demonstrated by observation. Its existence can only be inferred 
by analogy with areolar tissue: it will be remembered that it 
was proved to be present in the foetal condition of that tissue. 
The quantity of cytoblastema, in comparison to the fibres pre- 
sent, seems to me to be the principal distinction between areolar 
and fibrous tissue in the adult. The fibrous tissue contains 
2 great many more fibres within a given space than the areolar 
does, and they are not more minute than those of the latter 
tissue. There is just as great a difference, however, between 
fibres of areolar tissue taken from different parts of the body, 
as there is between the ordinary fibres of tendons and the most 
common form of areolar tissue, so that a very gradual transi- 
tion takes place. 


8. Elastic Tissue. The distinction between elastic and 
fibrous tissue is exhibited at a very early period. But my 
investigations into the history of the development of this tissue 
are very incomplete, and extended only so far as to render 
it probable that it presented no exception to the principle of 
development from cells. I made use of the aorta of a foetal 
pig and the ligamentum nuche of a foetal sheep for the purpose. 
The tissue taken from these two parts was very different in its 
general character. In a pig’s embryo, of six inches in length, 
the aorta had already acquired its yellowish colour and perfect 
elasticity. The external coat could be easily drawn off in 
long pieces, almost, indeed, as a distinct tube. Having drawn 
off a small portion of the middle coat (which, in order to avoid 
any suspicion of epithelium being mixed with it, was so care- 
fully done, that the internal surface of the vessel remained 


ES a ee 


ELASTIC TISSUE. 125 


uninjured), and torn it asunder a little, it was examined with 
the microscope; the first appearance presented was that of a 
great quantity of isolated cells, floating about in the sur- 
rounding fluid, each of which had its peculiar nucleus. (See 
plate III, fig. 12.) This easy separation of the cells is 
never seen in the same degree in the areolar and fibrous 
tissues, as they are there connected together by the cytoblas- 
tema, and by the tough fibres issuing from the cells. These 
cells of the coat of the aorta vary very much in shape. (See 
the figure.) Some are round, but most of them oblong, some 
terminate with a blunt extremity, others are acuminated on 
two or more sides, others again are prolonged into small pro- 
cesses, which again subdivide, but never extend to any great 
length. Many of them are somewhat compressed laterally. 
They all have a granulous aspect, but that appearance, so far 
as one can judge by rolling the cells about, seems to be refer- 
rible to the cell-membrane, and the contents appear to be 
transparent. The nucleus, enclosing one or two nucleoli, 
is attached to the interior of their walls. It is sometimes 
round, at others more or less elongated. These cells have 
become disengaged from the small portion of the coat of 
the artery before described. When the preparation itself is 
examined, many more cells are observed in it, and in addition 
to them, distinct elastic tissue, consisting of a network of 
minute, elastic, rough (?) (rauher) fibres, such as are found 
nearest to the internal coat of the aorta in the adult. (See 
Eulenburg, de Tela elastica, fig. 9.) It does not, however, 
present any fibres so thick as those which are found in the 
external layers of the same part. <A blighted nucleus may 
be recognized here and there in the network. What relation, 
then, do these cells bear to this still delicate, but so far as 
regards its characteristic features, perfectly-formed elastic 
tissue? Analogy would lead us to suppose them to be 
the primitive formation; I sometimes also thought, that in 
rare instances I could observe an immediate transition ; that 
I could see, for instance, one of these cells, furnished with a 
nucleus, pass continuously on one side into a small portion of 
reticular tissue, resembling in appearance the undoubted elastic 
tissue, whilst on its other side it retained its perfect cellular 
figure. But this occurred so rarely, that I am not enabled to 


126 ELASTIC TISSUE. 


state that a transition of these cells into elastic tissue was 
proved by observation. If, however, such be really the process 
of formation, as, from analogy, we are entitled to suppose, 
the bodies of these cells must then take a much more important 
share in the formation of the fibres than those of areolar tissue 
do, and the formation of the elastic fibres of the aorta holds a 
middle position between the generation of the horny fibres in 
the cortical substance of feathers (see p. 86, and pl. II, fig. 13) 
and the production of fibres in areolar and fibrous tissues. 
The reticular appearance of elastic tissue loses its singularity, 
when it is conceived to be generated in the same manner 


as those horny fibres in the feather, that is, partly by’ 


an elongation of the cells, and partly by a splitting of 
their bodies. The splitting of the elastic fibres is not to 
be regarded as an isolated phenomenon, since such division 
undoubtedly occurs in transitional stages in the development 
of all forms of areolar and fibrous tissue in the foetus. 
In this respect the elastic tissue seems to remain at a lower 
stage of development. Purkinje and Raiischel observed a 
darkish point in the centre of a transverse section of the 
elastic fibres of the aorta, and a dotted line m the course of 
the fibres, and thence inferred the existence of a rudimentary 
canal in their interior. This supposition, which I must 
confess formerly struck me as being a very bold one, has 
much more weight now, inasmuch as it is not improbable 
that all fibres which are formed by the prolongations of 
cells (even those of areolar tissue) are hollow, at least, that 
they are not composed throughout of one uniformly solid 
mass. If, as an observation of Valentin’s seems to indicate, 
still more minute fibres may be rendered visible by the aid 
of caustic potash in those of ordinary elastic tissue, I 
should be inclined to regard them as analogous to the primi- 
tive muscular fibres, whose signification, as we shall subse- 
quently see, differs entirely, in a morphological view, from the 
primitive fibres of areolar tissue. 

Whilst the elastic tissue of the aorta taken from a very 
young foetal pig exhibited in the manner before described the 
main characteristics of the tissue, namely, its yellowish colour 
and elasticity, the ligamentum nuche of a sheep’s foetus, at a 
much later period of gestation, was but very slightly developed. 


eS ee aes 


FIBRE-CELLS, ETC. 127 


It had a gray and translucent appearance, exhibited no elas- 
ticity, and when examined with the microscope, presented no 
trace of its future structure. A gray cord, indistinctly marked 
with longitudinal fibres, was seen, in which a great many 
cell-nuclei might be recognized. I did not prosecute any fur- 
ther researches, as the presence of the nuclei was sufficient 
proof that there was nothing essentially different in the type 
of its formation. 


On casting a retrospective glance over the class of fibre- 
cells which we have just been considering, we find that it 
forms a very natural and somewhat strictly defined group 
amongst the tissues. The tissues comprised in it are generated 
from nucleated cells, which are transformed into fasciculi of 
fibres by elongation, in the first place, and by the splitting of 
the bodies of the cells themselves into separate fibres at a sub- 
sequent period. The fundamental phenomenon previously 
described at page 39 is distinctly presented in the formation 
of these cells; a structureless, gelatiniform mass, the cytoblas- 
tema, is first present, and lies outside the cells already formed. 
The cells are developed in this, the nucleus being, in all pro- 
bability, the earliest formation. The growth of the cells 
proceeds, and they become transformed into fibres in the 
manner described. The quantity of the cytoblastema con- 
tinually diminishes in proportion to the cells or fibres which 
are forming, but probably part of it remains persistent be- 
tween the fibres throughout the whole of life; in the mature 
condition, however, it exists in greater quantity in areolar than 
in fibrous or elastic tissue. 

The mode of generation teaches us which parts of these 
tissues correspond to the constituents of those hitherto treated 
of. The elementary cells of areolar tissue, before undergoing 
change, correspond morphologically with the cartilage and 
epithelium-cells, the mucus-corpuscles, &c.; and as a fasciculus 
of fibres is generated from each cell of areolar tissue, a whole 
fasciculus of fibres of areolar tissue accordingly corresponds to 
what was an individual cartilage- or epithelium-cell, in the pre- 
vious classes. The structureless cytoblastema between the 
fibres of areolar tissue corresponds, however, to the firm inter- 
cellular substance, forming the principal mass of most cartilages, 


128 FIBRE-CELLS, ETC. 


or to the minimum of cytoblastema between the epithelium- 
cells ; or, lastly, to the fluid, in which the cells of the first 
class are formed. In this way one can also readily understand 
how fibro-cartilage forms a gradual transition between true 
cartilage and fibrous tissue ; it only requires that the cartilage- 
cells pass through the same transformations as the elementary 
cells of areolar tissue. 

As the present class was based upon the alteration in form 
which the cells comprised in it undergo, it necessarily could not 
present many modifications in the shape of the cells, and accord- 
ingly exhibits throughout merely an elongation of nucleated 
cells into fasciculi of fibres, and a subsequent splitting of the 
bodies into fibres. We have already seen the types of these 
changes in the second class, where the pigment-cells and those of 
the crystalline lens, &c., became elongated by a more vigorous 
growth of the cell-membrane at different spots ; and the class 
now before us merely affords us an instance of the same pro- 
cess in a higher degree, since here, one side of one of these 
more highly developed fibre-cells gives origin to a great num- 
ber, or even a whole fasciculus, of fibres. The cells of the 
cortex of the feather also furnished us with an example in the 
same class of the division of the body of the cells into fibres. 
Inasmuch as the prolongations of the pigment-cells remain 
hollow, however minutely they may ramify, one may suppose the 
same to be the case with regard to the fibres of the tissues 
now under consideration. The decision of this poimt would, 
as we shall subsequently see, be of great importance for 
the theory of nutrition; but it is quite impossible to deter- 
mine it by observation, in consequence of the cells of this class 
not possessing any characteristic contents like those of pigment. 
An observation by Purkinje and Raiischel was quoted, however, 
in favour of these cells being hollow. If the hollowness of the 
fibres of areolar tissue, &c., could be proved, there would 
then be a division of a single cell into many cells at each 
transformation of a fibre-cell, and thus the fibrous tissues would 
not lose their cellular character. 

The fibre-cells undergo chemical changes during their 
growth and gradual transformation into the fibres of areolar 
tissue, since that tissue, when boiled, even long after the forma- 
tion of fibres has commenced, yields no gelatinizing gelatine. 


; 


ELASTIC TISSUE. 129 


The formation of the fibres of areolar tissue from cells, having 
been typified already in the second class, it follows that orga- 
nization, or the presence of blood-vessels, does not establish 
any essential difference in the growth of the elementary par- 
ticles ; for this class belongs to the perfectly organized tissues, 
and areolar tissue is highly vascular. The unorganized tissues 
were formerly said to grow by apposition, and the organized 
by intussusception. We have already discussed this distinction 
at page 95. It is so far correct, that the young cells of unorga- 
nized tissues are not formed throughout the entire thickness of 
the tissue, but only in the neighbourhood of that surface, on 
which they are in contact with vascular substance, and where 
they therefore obtain the freshest cytoblastema. But if this 
distinction between the surface and parenchyma of the tissue 
be not present, in consequence of the blood-vessels being dis- 
tributed throughout its whole thickness, the young cells are 
then also generated in every part of the tissue ; and such is 
the case with areolar tissue. The primary distinction, there- 
fore, merely consists in the absence or presence of vessels, the 
difference in the place of formation of the new cells being but 
a secondary distinction. The elementary particles grow in both 
instances and by the same powers. We shall see hereafter how 
far the presence of vessels facilitates certain processes which 
occur during growth. The essential phenomena of growth, 
and, therefore, also the fundamental powers called into activity 
by it, are similar in both. But why a formation of vessels 
should take place in areolar tissue and not in epithelium, is a 
question for future discussion. 


CLASS V. 


Tissues, generated from cells, the walls and cavities of which 
coalesce together. 


The following is the type of formation in this class: inde- 
pendent cells, by which we mean such as have a special wall and 
cavity, are present in the first instance; these we shall call 
primary cells. They are either round or cylindrical, or of a 
stellate figure. When round or cylindrical, the primary 
cells are applied together in rows, the contiguous portions of 


9 


130 MUSCLE, 


the cell-walls then become blended, in such manner that 
merely simple septa remain, dividing each succeeding cell-cavity 
from its neighbour. These septa, however, become absorbed, 
so that the cavities of the different cells communicate. Instead 
of a number of primary cells, we then have one single long 
one, which we shall call a secondary cell. The cavity of such 
a one, therefore, consists of the united cavities of the original 
cells, and its cell-membrane of all their blended cell-mem- 
branes, except that the parts with which they were in contact 
are absorbed. The growth of the secondary cell proceeds 
like that of any simple imdependent cell. This appears to 
be the process of formation in muscle and nerve, so far, at 
least, as the observations, which will presently be communi- 
cated, extend. When the primary cells have a stellate figure, 
their bodies are not applied in rows, as in nerve and muscle, 
but are generated in larger imterspaces filled with cytoblas- 
tema or with cells of another kind. Their prolongations, 
however, come in contact, the walls coalesce at the points 
of junction, and the blended septa then become absorbed, so 
that the cell-cavities, which were at first separated, now com- 
municate. In this manner, when several prolongations of 
one cell come into contact with those of another, or of several 
others, we obtain, in the place of isolated, hollow, stellate cells, 
a network of canals, which are, in the first instance, somewhat 
thicker at the parts corresponding to the bodies of the cells, 
but become of pretty equal dimensions, in consequence of 
more vigorous expansion of the communicating prolongations. 
This appears to be the mode in which the capillary vessels are 
formed. The followmg detailed statement of observations 
upon the relation which muscles, nerves, and capillary vessels 
bear to elementary cells, will show how far the description 
just given, as the probable mode of formation, is to be regarded 
as proved by these, as yet, very incomplete researches, and will 
also indicate what deficiencies have yet to be supplied. 


1. Muscle. To ascertain the relation which this tissue 
bears to the elementary cells, we must have recourse to the 
history of its development. I was, unfortunately, prevented 
from investigating the earliest formation of muscular fibre, in 
consequence of not being able to obtain any very young 


MUSCLE. 131 


embryos ; but the deficiency in my researches may be sup- 
plied from the description given by Valentin (Entwicklungs- 
Geschichte, p. 268), from which the followmg passage is 
extracted: “ Long before separate muscular fibres can be dis- 
cerned, the globules of the primitive mass are seen, arranged 
in parallel lines, particularly when they are lightly pressed be- 
tween two pieces of glass. The granules then appear to he 
drawn somewhat nearer together, to become completely 
coalesced, in some situations, while at others the blending 
takes place only on the one or the other side, and to be com- 
bined into one transparent mass. In this way filaments are 
formed, which, in some situations, have an appearance like 
strings of pearls, at others, on the contrary, are less sharply 
indented ; they often also continue slightly puckered on one 
side, whilst the margin of the other has already become more 
straight. At a subsequent period, all trace of granules or 
division in the filament vanishes, and its outline becomes sym- 
metrically transparent and cylindrical. The muscular fibre 
usually undergoes no other change until the sixth month, ex- 
cept that its substance becomes somewhat darker and _ its 
cohesion closer. The first traces of transverse striz are ex- 
hibited in the sixth month. These fibres are the primitive 
fasciculi of muscle and not the primitive fibrils, which latter 
are formed by a splitting of the fasciculus into smaller fibres. 
From the period at which the muscular filaments become 
transparent and uniform, masses of globules, of a more or less 
spherical form and somewhat larger than the blood-corpuscles, 
begin to accumulate between them. They diminish again 
afterwards, and, blending with the gelatiniform mass which 
connects them, become converted into the connecting areolar 
tissue.” 

The youngest embryos in which I have investigated the 
generation of muscle were those of the pig, measuring three 
and a half inches in length. If a portion of one of the super- 
ficial dorsal muscles be removed from an embryo pig of that 
size, and examined under the microscope upon a black ground, 
a transparent gelatiniform mass is observed, in which parallel 
fibres (primitive fasciculi of muscle) run in close contact, 
having a whiter appearance than the surrounding gelatinous 
substance. As development proceeds, the transparent sub- 


132 MUSCLE. 


stance diminishes in quantity, the muscular fibres lie closer 
together, and have a more intensely white appearance upon 
the black ground. When some of this transparent substance, 
taken from a foetus of the size before mentioned (and in order 
to exclude as completely as possible the embryonal cellular 
tissue which surrounds the entire muscle, a portion should be 
cut out from the centre of the muscle), is examined with a 
magnifying power of 450, it exhibits various kinds of granules 
differing im size, and lying in a finely granulous mass. On 
examining these granules more accurately, they are found to 
vary, both in size and appearance, being round or oval, more 
or less opaque or transparent. A great many of them may be 
recognized as cell-nuclei by their form. In many instances, 
even when they are still connected together, the granulous 
substance around them is more or less distinctly seen to have 
a defined globular figure, within which the nucleus lies. This 
is, however, observed most distinctly when any of the granules 
become separated from the transparent substance, and float 
about in the fluid upon the object-glass. A quantity of globules 
are then seen floating about isolated, each one containing the 
characteristic cell-nucleus, which is placed eccentrical, varies 
much as to its size, and is often furnished with nucleoli. (See 
pl. III, fig. 13.) We are already familiar with this as the 
rudimentary form of most cells. The finely granulous_por- 
tion of the transparent mass is formed, in part, of the bodies of 
the cells, which, when in close contact, are difficult to distin- 
guish, and in part, of the cytoblastema in which the cells have 
been generated. Some of these cells which float about are 
becoming elongated into fibres, which are manifestly those of 
areolar tissue. Such instances, however, are rare, and these 
cells seem to be something quite peculiar. They might be re- 
garded as the primitive cells of new muscular fibres ; but from 
the manner in which Valentin expresses himself, one should 
infer that they are formed at a later period, for he says, 
“masses of globules begin to accumulate between the mus- 
cular fasciculi from the period at which they become trans- 
parent ;’ it is clear that he here refers to the nuclei of these 
cells. This must, therefore, remain an undecided point. 

We next examine the muscular fibres (primitive fasciculi) 
in the dorsal muscles of the same foetus. They do not all 


a Pres wy 


MUSCLE. 1383 


resemble one another in general character; some are more 
irregular, more granulous, whilst others are relatively smooth. 
The smoother ones represent cylinders, which are generally 
more or less flattened (see pl. IV, fig. 3), m which they are 
delmeated from the brachial muscles of a foetal pig seven inches 
in length, a representing the flat surface, 6 the marginal. The 
cylinder a@ presents a dark margin, and an internal clear por- 
tion, a distinction which is yet more manifest in c, where the 
dark margin is broader and sharply defined on its inner edge, 
so that it has quite the appearance of a hollow cylinder. 
I must, however, remark, that but very few fibres present this 
appearance sufficiently distinct to satisfy the mind of the ob- 
server. But in many instances it was so manifest, that no 
other explanation seemed left than to suppose the fibre a hollow 
tube. In the clear portion of the cylinder, which corresponds 
to the cavity, (in addition to some small granules,) larger oval 
corpuscles are seen, which are often very much extended in 
the longitudinal direction. Their form at once shows them 
to be nuclei, and they frequently contain one or two nucleoli. 
The distance at which they he from one another is more or 
less regular in different instances. They do not lie in the 
axis of the fibre, but eccentrically, upon and within the thick- 
ness of the wall, as is seen when the fibre rests upon its 
margin. (See the fibre 6.) That delineation exhibits a regu- 
larity in their position, since a nucleus lies upon the one side 
of the wall, the second on the opposite, and the third again 
upon the first side, and so on; such, however, does not appear 
to be the case in every instance. The nuclei are flat, for 
when viewed edgeways they have the appearance of mere 
stripes. The thickness of the wall of the cylinder seems 
to vary, as is shown by a comparison of a with c. The 
latter, c, the wall of which is the thicker, already presents an 
appearance of transverse strize. The nuclei, however, are also 
still visible in it, as well as small isolated globules which are 
contained in its cavity. Muscular fibre does not present any 
appearance of a cavity after the period of development before 
mentioned has passed, but the nuclei remain visible for a long 
time, lying in the thickness of the fibre, and often project 
upon the outside in the form of small prominences. 

The other form of muscular fibre is delineated in pl. IV, 


134 MUSCLE. 


fig. 1, from the dorsal muscles of a foetal pig of three inches 
and a half in length. They are in general somewhat thicker 
than those last described, more irregular, not so smooth, 
but more granulated. The existence of a special wall to the 
fibre and of a cavity in its interior, may be quite as distinctly, 
or even more clearly, recognised in many of these. (See the 
fibre ¢ in fig. 1.) The wall is not so smooth as in the other 
form of muscular fibre. The contents are always very granulous. 
Distinct cell-nuclei, and not unfrequently nucleoli also, even 
in the natural state, may often be perceived in them. Com- 
monly, however, only the circular or oval outlines of the 
nuclei are distinctly perceptible, m consequence of the other 
granules which are contained in the cavity of the fibre lying 
above them, and the general granulous nature of the fibre 
renders an accurate discernment of the nucleus particularly 
difficult. But if a drop of acetic acid be added, the fibre 
becomes perfectly transparent, and swells; the nuclei, on the 
contrary, remain dark, shrivel up slightly, and may then be 
distinguished with perfect accuracy. This is exemplified by 
fig. 2, which represents the fibre c of fig. 1 after having been 
treated with acetic acid. The indubitable cell-nuclei, partially 
furnished with nucleoli, are there seen, with isolated small 
dark granules between them. The nuclei have indeed under- 
gone a slight change from the acetic acid, but they do not all 
present a regular aspect even in the recent state. The 
majority of them are flat. In the recent state, some appear to 
be placed on their edges, presenting an appearance as though 
the cavity of the fibre were divided into compartments by 
small thick transverse striz. The nuclei he much nearer 
together in this than in the form of muscular fibre previously 
described, so that the distance of the central points of two 
nuclei from one another, is generally equal to, or even less 
than, the thickness of the fibre. 

This second form of muscular fibre appears to be an earlier 
condition of the first. The younger the embryo the more 
abundant is this form of fibre, and it gradually becomes less so 
as development proceeds. The steps of this transition may 
readily be conceived. The fibre becomes extended in its 
entire length, is thereby rendered thinner, the cell-nuclei 
are removed farther from one another, and in some instances 


MUSCLE. 135 


are also elongated m the direction of the fibre. Some 
of the nuclei, those for instance which appear to be placed on 
their edges, may possibly become absorbed at the same time, 
for they never present that position at a later period. The de- 
velopment of the whole cylinder proceeds simultaneously, its 
granulous aspect disappearing, and the small granules of the 
cavity also diminishing in quantity. All the stages of transi- 
tion from the second form into that first described may be 
observed. The extension does not appear to take place quite 
regularly, but may be stronger at particular parts, so that, 
for a considerable extent, a fibre may be somewhat narrow, 
and present no nucleus, and then again an intumescence oc- 
curs in which a nucleus lies. 

It now, however, becomes a question how the form of mus- 
cular fibre last described is generated, and what its elementary 
form may be. It presented a cylinder, which is most probably 
hollow, and may be presumed to be closed at both ends, since 
the muscular fibres terminate abruptly at the tendons, with a 
well-defined and bluntly-rounded extremity. Cell-nuclei le 
within this cylinder at very small distances from one another. 
Is the cylinder an elongated cell, in which nuclei are formed 
as the rudiments of new cells, which, however, are not deve- 
loped; or are the nuclei the remains of cells, which, by coales- 
cence with one another and absorption of the septa, form the 
entire fibre or cylinder? Or, in other words, is the fibre 
generated by a coalescence of cells ? 

I have not observed the stages of transition m which 
original cells arranged themselves in a linear series to form a 
fibre, the recent embryos at my command not being sufficiently 
young for the purpose. I have, indeed, met with an appear- 
ance in the form of muscular fibre last described, which might 
be regarded as an indication that those fibres are composed of 
small portions joined together. Their margins were incur- 
vated at different spots, and a line, indicative of a division, ran 
transversely across the entire thickness of the fibre. I have 
endeavoured to delineate this appearance in pl. IV, fig. 1, d, 
but I have not succeeded in representing its true character, 
and it was not, in itself, conclusive. There are some other argu- 
ments in favour of the fibre of muscle being composed of sepa- 
rate particles. Many of the muscles of fishes or tadpoles, for 


136 MUSCLE. 


instance, when simply torn, separate into microscopic particles, 
which have an almost similar length. The same takes place, 
according to C. H. Schultz, during the digestion of muscle in 
the stomach, and, according to Purkinje, in muscle which is 
exposed to the action of an artificial digestive fluid. The 
observations of Valentin, already mentioned, admit, however, 
of no other explanation than that previously given ; and the 
history of the period of formation deficient in my researches 
(from the cause before stated) may be completed from his. 
According to him, “ globules of the primitive mass, arranged 
longitudinally, in a linear series, are present previous to the 
muscular fibres. The granules, then, seem to draw somewhat 
nearer together, and to coalesce, at some parts completely, at 
others, on the contrary, only on the one or other side. In 
this manner threads are formed, which present at some spots 
the appearance of strings of pearls, whilst at others they 
are less sharply indented ; they are also often seen to be still 
wrinkled on one side, while on the other their margin is 
already nearly a straight lime. The expression “ granules of 
the primitive mass” (Urmasse), or other similar terms, have 
been hitherto used to denote either the elementary cells 
themselves or their nuclei, indiscriminately ; mm consequence 
of the distinction between them, and their relation to each 
other being unknown. In the passage quoted, Valentin can- 
not have meant the nuclei, for, as we have seen, they do not 
coalesce. What he calls globules of the primitive mass must, 
therefore, be the elementary cells furnished with their nuclei, 
and in their earliest stage of development ; that is, before they 
have undergone any transformation. ‘The following arguments 
may likewise be adduced in favour of the correctness of the 
explanation which assumes these “ globules of the primitive 
mass” to be cells. In the first place, the structure formed by 
their coalescence, namely, the primitive fasciculus of muscle, 
is hollow; and, secondly, in the early stage of development of 
the fasciculi, the cell-nuclei lie just so closely together, as 
they would if each nucleus had pertained to a previously round 
cell. If these nuclei were subsequent formations, generated 
in the primitive fasciculus of muscle, as in a cell, they ought 
to be more numerous in old than in young muscles. 

It, therefore, seems scarcely to admit of a doubt, that 


MUSCLE. 137 


each primitive muscular fasciculus is a secondary cell, formed 
by the coalescence of primary round cells, each furnished with 
a nucleus, and which were arranged together in a row. After 
the coalescence of the contiguous portions of the cell-walls has 
taken place, an absorption of the septa remaining between the 
cavities of the two neighbouring primary cells must commence, 
since no such septa can be perceived within the secondary cell 
at a later period. If the little transverse striz, by which the 
cavity of the fibres is sometimes divided, be actually nuclei 
placed transversely upon their edges, they are probably such 
as lay upon that part of the wall of the cells which was ab- 
sorbed. It seems that the coalescence of the cells, however, 
is not sufficiently complete to prevent a separation taking 
place more readily at the points of junction than elsewhere, 
and on this the phenomena of the artificial division of muscle 
before mentioned probably depend.’ 

When I made my first communication upon the formation 
of the primitive fasciculi of muscles by the coalescence of cells 
(Froriep’s Notizen, No. 103), the only corresponding instances 
known to exist among vegetable cells were those of the spiral 
and lactiferous vessels. The interest attached to the subject 
has very much increased since Meyen’s discovery of a much 
more striking analogy in the cells of the liber or inner bark 
—(bastzellen). (Wiegmann’s Archiv, 1838, p. 297.) He found 
that these long-extended cells, when boiled in hydrochloric 
acid, fell into small particles of nearly equal length; and 
investigation into the development of the cells of the liber in 
buds showed, that m the early period a corresponding quantity 
of distimct, somewhat longitudinally extended, prismatic, pa- 
renchymal cells are present, which are placed with their 
extremities accurately arranged one upon another, that they 
unite together at those parts, and that their septa are after- 
wards absorbed. 

The secondary muscle-cell passes subsequently through all 
the changes incident to a simple cell. Its wall is at first thin, 


' It might be important to examine whether the zigzag plications of muscles, 
during contraction, have not perhaps some connexion with the length to which the 
portion of a muscular fibre generated from one single cell has become expanded, so 
that probably the angle of each flexion coincides with the point of junction of two 
cells. 


138 MUSCLE. 


and it contains many small granules in its cavity in addition 
to the nucleus. <A transformation of the cell-contents then 
takes place, the granules gradually disappearing ; the wall of 
the cell at the same time becoming thicker at the expense of 
the cavity, so that eventually the latter completely disappears, 
and the entire secondary cell is converted into a solid cord. 
The cell-nuclei at first remain whilst this thickening of the 
cell-wall is going on, and become enclosed by it, rather than 
pushed into the cavity of the cell. They are at length entirely 
absorbed. Is, then, the thickening of the wall of the secondary 
muscle-cell a thickening of the cell-membrane itself, as ap- 
peared to be the case in cartilage? or is it a secondary deposit 
upon its inner surface, so that the cell-membrane is chemi- 
cally and microscopically distinct from the substance, by 
means of which the secondary cell becomes converted into a 
solid cord? The latter is the more usual case in vegetables. 
The position of the cell-nuclei affords important evidence for 
the solution of the above question ; for as those bodies, gene- 
rally at least, lie firmly attached to the imner surface of the 
cell-membrane, they would be pushed towards the interior by 
a thickening of the cell-membrane itself, whilst a secondary 
deposit upon its mner surface, must enclose and fix them 
there, unless they should become separated altogether from 
the cell-wall. Now, in muscle, they actually remain lying 
in the circumference of the fasciculus, as represented by 
pl. IV, fig. 3, 6. This fact, then, renders it probable that the 
thickening of the wall of the secondary muscle-cells is due only 
to a secondary deposit. Such a supposition must, however, have 
been adopted, independent of the argument just raised, since 
the muscular fasciculi are, as it seems, enclosed by a struc- 
tureless membrane. The fasciculi have been long described as 
invested by a sheath, but that investment has been considered 
to be composed of cellular tissue, and to correspond in the 
primitive fasciculi to the cellular tissue, by which the larger 
fasciculi are separated from one another. This membrane 
seems, however, to have quite a different signification, and 
to be the cell-membrane of the secondary muscle-cell. It 
is structureless, very transparent, and appears as a very narrow 
and sharply-defined border around each primitive fasciculus. I 
well know how readily such an appearance is produced by a 


MUSCLE. 139 


mere optical deception, and that one can never be positive 
with respect to it unless it be observed that the margin in 
question does not accurately follow every bend of the fasci- 
culus. It is, therefore, difficult to be convinced of this in 
mammalia; but in all those larve of insects which present 
the broad transverse strize of the fasciculi, discovered by 
Miller, the membrane, when the continuity of the proper 
muscular substance of a primitive fasciculus has been broken 
at a certain point, may be distinctly observed passing over 
uninterruptedly from the one portion to the other. Pl. IV, 
fig. 4, represents such a fasciculus ; the membrane encompasses 
it so loosely (this larva had been preserved in spirits of wine) 
that a portion of the muscular substance could even change 
its position within the cavity. The membrane, where entirely 
isolated from the other parts of the preparation, shows itself to 
be quite structureless, and, indeed, the sharply-defined ex- 
ternal contour renders it very improbable that it should be 
composed of areolar tissue. I, therefore, consider it extremely 
probable that it represents the cell-membrane of the secondary 
muscle-cell. It thus not only serves to isolate the fasciculi, 
but forms an essential constituent part of them. Pl. IV, 
fig. 5, exhibits this structureless membrane upon a muscular 
fasciculus of the pike; this preparation, however, was not 
quite convincing, inasmuch as the inferior edge of the fasciculus 
was covered by muscles lying above it. By means of this mem- 
brane, the muscular fasciculus remains, throughout its entire 
existence, a cell with a closed membrane and a cell cavity, 
the latter being filled with a firm substance, the peculiar 
muscular substance. It, therefore, clearly follows from the 
above that nervous fibres cannot pass between the primi- 
tive fibres (fibrils) of muscle ; and that the latter cannot 
separate from their fasciculi, so as to pursue a more extended 
and independent course, as is common with fibres of areolar 
tissue ; since, in either case, the cell-membrane must be 
ruptured. 

The true muscular substance, which is thus, in the first 
place, formed as a secondary deposition upon the inner surface 
of the secondary muscle-cell, and continues to be so deposited 
until the entire cavity is filled, is composed in its mature con- 
dition, of very minute longitudinal fibres, the so-called primi- 


140 MUSCLE. 


tive fibres (fibrils) of muscle. These longitudinal fibres do not 
appear to represent the original condition of the secondary 
deposit, but the latter is structureless at first, and its trans- 
formation into fibres takes place subsequently. The change 
seems, however, to commence at a very early period, and 
indeed before the cavity is completely filled. The transverse 
strie of the muscular fasciculi, which, according to my mode of 
explanation, are produced by the peculiar form of the primitive 
fibres, likewise make their appearance before the complete 
filling up of the cell-cavity, as pl. IV, fig. 3, c, exhibits. 

According to the observations of Meyen on the formation 
of the cells of the liber, after the coalescence of the cells and 
absorption of the septa, a secondary deposit also takes place 
upon the common cell-membrane in the same way that we 
have observed to take place in muscle; but I know of nothing 
amongst vegetables analogous to a secondary deposit consisting 
of longitudinal fibres. On the contrary, according to Valentin, 
such deposits appear to take place in plants universally in 
spiral lines. The beaded appearance which the primitive mus- 
cular fibres here and there present, might perhaps be regarded 
as the result of this tendency to a spiral formation, the intu- 
mescences (beads) being so placed, as to produce the transverse 
strie, and the latter may perhaps be spiral and not circular. 
This is, however, a mere conjecture, and requires further re- 
search. 

The involuntary muscles, such as do not present the trans- 
verse striz, appear to originate in a manner similar to that 
just described. They differ, however, from the voluntary or 
striated muscles, in their fibres being generally shorter than 
those of the latter; probably, therefore, fewer primary cells 
arrange themselves together to form a secondary cell, and 
their fibres are commonly thinner and flat. I found im a 
human uterus, which contained a mature foetus, some long 
muscular fibres of the breadth of the common primitive fasci- 
culi of voluntary muscles, which were so flat as scarcely to 
amount to 0:0010 to 0:0015 of a line in thickness. The 
involuntary muscles, likewise, have cell-nuclei, proving that 
the fibres composing them do not correspond to the primitive 
fibres (fibrils), but to the primitive fasciculi of the voluntary 
muscles. An opposite view of the matter might be taken 


NERVES. 141 


from the circumstance of their frequently exhibiting no trace 
of longitudinal striz, and that probably the greater portion of 
them do not contain other more minute primitive fibres, or 
at least only such as are imperfectly developed. In this re- 
spect they are not so highly developed as the voluntary 
muscles. Perhaps the peculiar secondary deposit upon the 
cell-membrane of the secondary cell is all that is essential to 
the contraction of muscle; and it may not be important that 
that substance should consist of minute longitudinal fibres. 

In order briefly to recapitulate our researches into the 
generation of muscle, the process may be thus stated. Round 
cells, furnished with a flat nucleus, are first present, the 
primary cells of muscle. These arrange themselves close 
together in a linear series; the cells thus arranged in rows, 
coalesce with one another at their points of contact ; the septa, 
by which the different cell-cavities are separated, then become 
absorbed, and thus a hollow cylinder, closed at its extremities, 
the secondary cell of muscle, is formed, within which the 
nuclei of the original cells, from which the secondary cell has 
been formed, are contained, generally lying near together on 
its wall. This secondary cell, then, passes through all the stages 
of a simple one. It expands throughout its entire length, 
whereby the nuclei are farther removed from one another, 
and sometimes even become elongated in the same direction. 
A deposit of a peculiar substance, the proper muscular sub- 
stance, takes place at the same time upon the inner surface 
of the cylinder, by which the cavity is at first narrowed, and 
at length completely filled. The cell-nuclei lie external to this 
substance, between it and the cell-membrane of the secondary 
cell. 

The transverse striz in the voluntary muscles become more 
manifest, and the deposited substance is more distinctly seen 
to be composed of longitudinal fibres, as the foetus advances 
in age. ‘The nuclei are gradually absorbed. The cell-mem- 
brane of the secondary muscle-cell remains persistent through- 
out life, so that each primitive muscular fasciculus is always to 
be regarded as a cell. 


2. Nerves. The nervous system presents two forms of 
elementary structure: Ist, fibres, nervous fibres in the ex- 


142 NERVOUS FIBRES. 


tended sense of the term, including the fibres of the brain and 
spinal cord: 2d, globules, ganglion-globules, in addition to the 
ganglia occurring in the brain and spinal cord. Our task is 
to point out the relation which these two forms of elementary 
structure bear to the elementary cells. 


Nervous Fibres. 


Of these, there are two different forms: a, the common 
white nervous fibres ; 4, the gray, so-called organic fibres. 


a. White nervous fibres. They have the appearance of 
fibres, which, when examined microscopically, exhibit very 
dark margins, and these margins are produced by a substance 
apparently identical with that which gives them their white 
colour when examined with the unaided eye. Since the cause 
of this colour does not appear to be situated in the whole 
fibre generally, but to be confined to its external portion, this 
latter may be termed the white substance of the nervous 
fibres. The margin of a fibre generally presents a double 
outline on both sides, so that it has the appearance of a 
hollow tube, and the distance between the two outlines, 
then, denotes the thickness of the white substance. According 
to the researches of Remak, the white substance of every 
nervous fibre may be removed by pressure, and an extremely 
pellucid, pale band, which was previously surrounded by 
the white substance, then remains, corresponding to that 
which, previous to the manipulation, seemed to be the contents 
of the tube. (See R. Remak, Obss. Anat. et Microsc. de Syst. 
Nerv. Struc., Berol. 1838.) 

Two opinions with respect to the nervous fibres may be 
deduced from the above observations ; either this pale band is 
the proper nervous fibre, and the white substance only a 
sheath (cortex) around it (this is the view taken by Remak), 
or the nervous fibre is actually a hollow fibre, the wall of 
which is formed by the white substance, the contents of which, 
however, are not fluid, but composed of a tolerably firm sub- 
stance, namely, the above-mentioned band. 

The history of the development of the nervous fibres must 


NERVOUS FIBRES. 148 


explain the relation which they bear to the cells. Remak! 
describes the early condition of the nerves in the following 
manner: ‘The substance of the cerebro-spinal nerves of the 
rabbit, in the third week of embryonal existence, consists of 
corpuscles, some of which are irregularly spherical, others 
slightly elongated, having a very delicate filament adhering 
to them; they are mostly transparent, and arranged in rows 
without, however, presenting any distinctly perceptible fibrous 
structure.” And l. c. page 153, he says, “ A structureless and 
general globular mass is the original form, from which the 
primitive fibres of the cerebro-spinal nerves are developed. 
These primitive fibres are at first varicose, and contain no 
medulla; most of them pass into the cylindrical form, through 
the intermediate stage of transitional fibres.” 

I have investigated the development of nerve in the feetal 
pig. The nerves of the foetus have not the shining white 
colour, presented by those of the adult animal, but are gray 
and transparent, and the younger the embryo the more strik- 
ing are these appearances. We are, therefore, quite prepared 
to find that microscopic imvestigation shows the white sub- 
stance of the fibres to be less perfectly or not at all developed. 
If a nerve, taken from a foetal pig of about six inches in length, 
be spread out, in the usual mode of preparation by tearmg it 
under water, some fibres are seen which very much resemble 
those of the adult animal, and which are furnished with 
outlines almost as dark. The greater part of the substance, 
however, does not form connected fibres, but consists of separate 
round globules, or more or less long, irregular little cylinders, 
arranged with their long axes in the direction of the course of 
the nerves, having outlines, however, quite as dark as those of 
the nervous fibres. These appear to be what Remak refers 
to in the description previously quoted. In addition to them, 
however, a substance of quite another appearance is seen, 
which has not the dark outline, does not appear pellucid but 
granulated, and in which the cell-nuclei are distinctly recog- 
nisable. 

When the other constituent parts predominate, the nuclei 


1 Miller’s Archiv, 1836, p. 148. Respecting the microscopic structure of the 
brain and spinal cord of the foetus, see Valentin, Entwickelungsgeschichte, p. 183. 


144 NERVOUS FIBRES. 


may very probably be overlooked, or possibly be regarded 
as extraneous substances. But they are in fact the primitive 
structure of nerve, for the younger the foetus the greater is 
their relative quantity, and in a pig’s foetus of three inches in 
length, I found them the sole constituent of nerve, none of 
the fibres furnished with the dark margins, nor any of the 
cylinders or globules being visible at that period of deve- 
lopment. The development of nerve, however, does not appear 
to proceed uniformly in all individuals; for the dark globules 
and cylinders were already present in some other pigs’ em- 
bryos, which were scarcely any larger. Pl. IV, fig. 6, repre- 
sents a portion of the ischiatic, and fig. 7, of the brachial 
nerve of such a foetus. We observe a palish, and very 
minutely-granulated cord, which, in consequence of certain 
longitudinal shadings, such as the delineation exhibits, pre- 
sents the appearance of a coarse fibrous structure. Round or 
for the most part oval corpuscles, which are immediately recog- 
nised as cell-nuclei, and which sometimes also contain one or 
two nucleoli, are generally seen in the course of these shaded 
parts, throughout the entire thickness of the cord. Sometimes 
a fibre separates from such a cord, and stands out isolated, as 
at a in both the figures, and the nuclei are then seen to lie in 
the course of the fibres. <A single fibre presents several nuclei 
in its course, as was also observed in secondary muscle-cells 
(see fig. 8, 4), but I have never remarked it in the cells of the 
fourth class, the fibre-cells. Although the (nervous) fibres 
cannot at this early period be distinctly perceived to be hollow, 
the wall not being distinguishable microscopically from the 
contents, yet we shall see that the progress of development 
renders it highly probable that they are so. If then these 
(nervous) fibres are so far analogous to the early condition of 
secondary muscle-cells, that they are hollow, and in various 
parts of their course contain nuclei, whose form shows them 
to be ordinary cell-nuclei, it is probable that they are gene- 
rated in a similar manner to muscle; that is, that they are 
formed by the coalescence of primary cells, to which the nuclei, 
just noticed as present upon the fibres, have pertaimed; so 
that thus the nervous fibres would be secondary cells, cor- 
responding to the secondary muscle-cells, or primitive muscular 
fasciculi. The actual observation of the primary cells of nerve 


NERVOUS FIBRES. 145 


in their independent state, is very difficult, from the circum- 
stance of our being unable at that period to distinguish between 
them and the surrounding tissues ; for a whole organ is then 
composed entirely of independent cells, which have not as yet 
undergone any transformation. It is true I saw an independent 
cell, furnished with a nucleus, which seemed to have separated 
from the nervous cord, in one of the preparations alluded to, 
fig. 6 6; but I cannot positively assert that it had actually 
separated from that particular part, nor that it was a primary 
nerve-cell, for the cells in that preparation had not as yet un- 
dergone any change. In this instance, therefore, we must 
content ourselves, for the present at least, with the analogy to 
muscle. 

These fibres, or secondary nerve-cells, differ very much in 
their appearance from the subsequent nervous fibres, which are 
furnished with distinct but not dark outlines; they have a 
pale, granulated aspect. By progressive development, however, 
they become converted into the white fibres, and pl. IV, 
fig. 8, d, represents the transition. The part of the figure 
to the right hand exhibits the fibre yet in the early condition, 
pale, granulated, and furnished with a cell-nucleus ; in the 
portion to the left, it has completely assumed its subsequent 
form: it has a dark outline, is not granulated, and the one 
portion passes immediately into the other. The identity be- 
tween these pale fibres and the subsequent white nervous fibres 
is thus established. 

In what then does this transformation of the pale granu- 
lated fibres into the white fibres consist? Clearly in the 
development of the white substance ; we may, however, imagine 
three different modes in which this development may take 
place. It may take place, Istly. By the white substance form- 
ing as a sheath (cortex), around each fibre, and in this manner 
enclosing it. By this mode of explanation the fibre would be 
identical with the pale band discovered by Remak, which 
would therefore be the cell-membrane itself. 2dly. The white 
substance might be regarded as a transformation and thicken- 
ing of the cell-membrane of those fibres, or secondary nerve- 
cells. According to this view, the white substance would be 
the cell-membrane, and Remak’s band the firm contents of the 
secondary cell. 3dly. The white substance may be formed as 

10 


146 NERVOUS FIBRES. 


a secondary deposit upon the inner surface of the cell-mem- 
brane, being chemically distinct from the latter, and the 
remainder of the cell-cavity may then, and not until then, be- 
come filled up by Remak’s band. 

It will be seen that the above question is analogous to 
that raised when we were treating of muscle, viz., whether 
the proper muscular substance be a thickening of the original 
eell-membrane itself, or a secondary deposit upon it. The 
reply is not, in either instance, essential to the proof of the 
origination of nerves or muscle from cells, but it is of so much 
the more importance for the explanation of the structure of a 
perfectly-developed nerve. If any conclusion may be drawn 
from the few observations which I have made on this point, 
the latter view appears to me the most probable, viz., that 
the white substance is a secondary deposit upon the inner 
surface of the cell-membrane. The white substance of each 
nerve is surrounded externally with a structureless and peculiar 
membrane, which appears to be minutely granulated. This 
membrane presents itself as a narrow, clear border, which 
is readily distinguished from the dark contours of the white 
substance. This membrane seems hitherto to have been in- 
cluded with the neurilema or with the cellular tissue, which 
surrounds the nervous fibre, and although its external outline 
is generally very sharply defined in the nerves of the frog, it 
would be difficult, on examination of the entire nerve of a 
mammal, to arrive at any conviction of its distinct and sepa- 
rate existence, did not opportunities of observing it im an 
isolated state present themselves. Pl. IV, fig. 9 a, represents 
such a preparation, taken from the cranial portion of the 
nervus vagus of a calf. The continuity of the white substance 
has here been broken by the process of preparation; but 
where it still exists, the double contours, (and thus the thick- 
ness of the white matter), may be clearly distinguished. But 
the nerve still exists at the part where the white substance is 
separated, its sharply-defined external margins may be seen, 


although their contours are but pale, and it may be observed 


that this pale outline does not pass into the external dark 
one of the white substance, but is continued on the outside of 
it as a narrow border, parallel to the two outlines of the 
white substance. The white substance of nerve is, therefore, 


f 
« 
Ps 
, 


NERVOUS FIBRES. 147 


surrounded externally with a thin, pale membrane, which has 
a sharply-defined external margin. If the membrane be very 
thin, it cannot be recognised as the pale border round the 
nervous fibre ; it is still, however, distinctly visible at situations 
where the white substance is destroyed. (See fig. 9 6.) The 
mere fact of the membrane possessing a defined external 
border, is evidence against its being composed of areolar tissue ; 
and even the portion which does not contain any white substance, 
presents no appearance of a fibrous structure ; it simply ap- 
pears to be somewhat minutely granulated. If this be correct, 
the membrane can have no other signification than that of 
cell-membrane of the nervous fibre, or secondary nerve-cell. 
The white substance is then a secondary deposit upon its 
inner surface. The position of the cell-nuclei is also favorable 
to this view. Most of the cell-nuclei, presented by the nervous 
fibres in their earliest and as yet pale condition, disappear 
during the formation of the white substance, a circumstance 
which is common to most other cells. Some, however, appear 
to remain for a longer period ; occasionally, although rarely, a 
cell-nucleus is here and there seen upon the side of a nerve, 
(the white substance of which is completely developed), lying 
m the pale border, which surrounds the white substance. 
Fig. 9, ¢ and d, exhibits them from the nervus vagus of a 
ealf. At ¢ the white substance, corresponding to the nucleus, 
even forms a slight projection into the cavity of the fibre. 
This nucleus seems therefore actually to belong to the fibre, 
and to lie upon the inner surface of the cell-membrane, while 
the white substance is so deposited, that the nucleus remains 
situated external to it. The band discovered by Remak would 
then be the proper cell-contents. Meanwhile I beg that the 
above may be regarded simply as an attempt at an explana- 
tion, the accuracy of which must be decided by further 
researches, for much more extensive investigations and a sepa- 
rate and distinct consideration are absolutely necessary for 
aceurate decision of so important a subject. 

According to the foregoing explanation, therefore, each 
nervous fibre is, throughout its entire course, a secondary cell, 
developed by the coalescence of primary nucleated cells. 
With respect to these cells, we remark, Istly. An external, pale, 
‘thin cell-membrane, having a granulated but not a fibrous 


148 NERVOUS FIBRES. 


aspect, the inner surface of which constantly exhibits cell- 
nuclei in the very early period of the development of nerve ; 
but in the somewhat more advanced stage, when the white 
substance is developed, they are only occasionally found. 2dly. 
That the white, fat-like substance to which the peculiar appear- 
ance and distinct outline of the nerves are chiefly referable, is 
deposited upon the inner surface of this cell-membrane. When 
this deposit is thick, its double contour (to which the nerve is 
indebted for its tubular appearance), may be recognised ; this, 
however, escapes observation when only a thin stratum of 
white substance is present. Morphologically considered, it 
therefore corresponds to the peculiar substance of muscle, for 
that is likewise developed asa secondary deposit upon the mem- 
brane of the secondary muscle-cell. 3dly. That the rest of the 
cell-cavity appears to be filled up by a firm substance, namely, 
the band discovered by Remak. There seems to be no struc- 
ture analogous to this band in perfectly-developed muscles, for 
there, the secondary deposit, that is, the formation of the pro- 
per muscular substance, proceeds until the cavity of the 
secondary cell is completely filled. 

We have thus traced the development of nerve to its per- 
fect state, without those irregular globules and little cylinders 
with the dark outlines, (which were mentioned at page 143, 
as occurring at a middle stage of the development of nerve 
in addition to the pale fibres and the matured nervous fibres), 
having proved to be a transitional step in the process. I 
am inclined to regard them as an artificial product, caused 
by pressure and the action of water upon the as yet very 
delicate nerve. If, for example, water penetrate through the 
cell-membrane by imbibition, the oil-like white substance re- 
tracts into separate rounded bodies, and the facility with which 
this takes place is proportionate to its slight degree of con- 
sistence. This is often seen even in fully-developed nerves ; 
an entire nerve frequently separates from this cause into sepa- 
rate globules or little cylinders, which have sharply-defined 
outlines, so that merely the cell-membrane proceeds uninter- 
ruptedly, in the form of a pale stripe, from the external wall of 
one of the dark portions to that of the other. Valentin has 
given a delineation of such a state of the nervous fibre, (Acta 
Acad. Leopold. Nat. Curios. vol. xviii, pl. III, fig. 7). As the 


a I Ce ye aay mee Wier Senr— « 


NERVOUS FIBRES. 149 


white substance is less consistent in the foetus, it separates the 
more readily, and the artificial generation of such globules is 
very easy of observation in foetal nerves. 

The growth of nerves neither proceeds from the circum- 
ference towards the central organs, nor vice versd, but their 
primary cells are included amongst those from which every 
organ is formed, and which, so far at least as their appearance 
is concerned, present no marks by which they can be distin- 
guished from other cells. They are first characterized as 
nerves, when they become arranged in rows and coalesce to 
form a secondary cell. After that coalescence each nervous 
fibre forms a separate cell, which pursues an uninterrupted 
course from the organ, in which its peripheral extremity is 
situated, to the central organ of the nervous system. The 
white substance of nerves does not appear to be formed at 
so early a period in their peripheral extremities, as it is in 
their trunks. The Medizinischen Zeitung for August 1837, 
contains a description which I gave of some nerves from the 
tail of frog’s larvee, which presented an appearance quite dif- 
ferent from ordinary nerves, inasmuch as they had a pale con- 
tour-and no perceptible cavity. They were nerves in an early 
stage, previous to the development of the white substance. 
They represent the only form of nervous matter which we find 
in the tail of very young larve. Some isolated nerves, having 
the ordinary appearance of the dark contours, gradually make 
their appearance, and afterwards increase in quantity; they 
were first observed in the neighbourhood of the muscular fas- 
ciculus which traverses the middle of the tail. The development 
of the white substance appears therefore to advance from the 
trunks towards the circumference. These white fibres become 
more minute and paler towards the periphery. Sometimes such 
a fibre seems to terminate suddenly with even an incomplete 
acumination. But, on a more accurate observation, some ex- 
tremely delicate, very thin filaments are generally seen going 
off from it. The pale immature fibres in the tail of the 
frog’s larvee also subdivide. A question now arises are those 
more minute fibres (which at least present an appearance of 
subdivision) already prepared within an ordinary white primi- 
tive nervous fibre, or are they actual subdivisions? Since each 
nervous fibre is a secondary cell, and retains its character as 


150 NERVOUS FIBRES. 


a simple cell, and since the simple cell-membrane continues to 
exist distinct from its secondary deposits, and from the cell- 
contents, it is quite conceivable that fibres may be generated 
in the secondary deposits or in the cell-contents, as they are in 
muscle, although we have as yet no evidence of the fact; but 
these fibres could no more issue out free from the white 
nervous fibre, than the primitive fibres of muscle could from 
secondary muscle-cell, because, in order to do so, they must 
necessarily rupture the cell-membrane of the secondary cell. 
These subdivisions, therefore, so far as the structure from 
whence they issue corresponds to an ordinary nervous fibre, 
and is not merely a fasciculus of very minute secondary nerve- 
cells, cannot be a mere appearance, nor anything but actual 
divisions, a simple secondary nerve-cell becoming eiongated into 
several minute fibres, in a manner analogous to that which 
we have witnessed in the fibre-cells, (see page 115.) The 
nerves in the tail of the tadpole may therefore be described 
as terminating by the nervous fibres, that is, the secondary 
cells becoming split in different directions after the manner 
of fibre-cells or stellate cells. In the memoir before alluded 
to, I have noticed some swellings upon the pale nervous fibres 
in the tail of the tadpole. They have a double signification ; 
some which are marked off from the rest of the fibre by a 
sharply-defined outline are the nuclei of the cells, from which 
the fibres have been generated ; the majority, however, which 
pass into the fibre without a well-defined contour, as gene- 
rally occurs at situations where the fibres divide and diverge 
towards different sides, are the bodies of the original cells, 
which (especially when they become elongated at different 
parts into fibres) remain somewhat thicker than the prolonga- 
tions themselves; the pigment-cells, pl. II, fig. 9 a, exhibit 
this appearance. 


b. Gray or organic nervous fibres. The gray cords, which, 
according to the researches of Retzius and J. Miler, are derived 
from the sympathetic nervous system, and mingled with the 
cerebrospinal nerves 11 which they sometimes pursue a long 
isolated course, owe their gray appearance, according to the in- 
vestigations of Remak, “to the peculiar structure of the primi- 
tive fibres, which arise in the ganglia. They are not tubular, 


NERVOUS FIBRES. 151 


that is, surrounded with a sheath, but naked, being transparent, 
almost gelatious, and much more minute than most of the 
primitive tubes. They almost always exhibit longitudinal lines 
upon their surface, and readily separate into very minute fibres. 
In their course they are very frequently furnished with oval 
nodules, and covered with certain small oval or round, more 
rarely irregular, corpuscles, which exhibit one or more nuclei, 
and in size almost equal the nuclei of the ganglion-globules.” 
(Observationes anat. et microsc. de system. nervos. structura. 
Berol., 1838, p. 5.)' 

These corpuscles may at once be recognised, both in Remak’s 
delineations, and when examined in the natural state, to be cell- 
nuclei, which are round or oval, and frequently furnished with 
one or two nucleoli. They are attached to the most minute fibres, 
and as they are thicker than the fibres, they often appear to be 
situated only on their outside. Observation, however, does 
not warrant the conclusion that such is actually the fact. In 
the secondary muscle-cells (in which the nuclei decidedly lie 
within the cell) it frequently appears, and especially in the 
later periods of development, previous to the disappearance of 
the nuclei, as if the nuclei lay externally to the cell, inasmuch 
as they become pushed towards the outside. But no doubt 
the cell-membrane is at the same time elevated upon them, as 
we saw to be so distinctly the case in the fat-cells. (Pl. ITI, 
fig. 10.) Now, these most minute organic fibres, furnished 
with nuclei, precisely resemble the earlier condition of the 
white nervous fibres, as they were represented in pl. IV, fig. 
8, a 6. Both have the same pale, minutely-granulated ap- 
pearance, and both present cell-nuclei in their course. The 
only difference is, that the organic fibres are much more 
minute and the nuclei smaller. Each single nucleated or- 
ganic fibre (I do not mean an entire fasciculus of them) cor- 
responds to a white primitive fibre, and is probably, like it, a 
secondary cell, which has been generated by a coalescence of 
primary cells, whose nuclei are the nodules described by 


' Remak’s discovery of the peculiar structure of the organic nervous fibres ex- 
plains an observation previously communicated by me upon some extremely minute, 
pale, nervous fibres, which did not appear tubular, and were nodulated at different 
spots, and which I discovered in the mesentery of frogs. No doubt they were or- 
ganic fibres. 


152 GANGLION-GLOBULES. 


Remak as existing upon these fibres. The similarity between 
the organic fibres and that which I have described as the 
earlier condition of the white nervous fibres, might be adduced 
as an objection to my description of the formation of nerves, 
and it might be said, that that form seemed to be the earlier 
form of the white nervous fibre, because the organic nerves 
were developed earlier than the white, and, therefore, organic 
fibres were the only ones present in the first instance. Ob- 
servation of the actual transition, as represented in pl. IV, 
fig. 8, ec d, would, however, refute this argument. Each pale, 
nucleated fibre becomes a white nervous fibre, as an immediate 
consequence of the formation of the white substance, which 
is probably a secondary deposit upon the internal surface 
of the hollow fibre. The formation of this white substance, 
which, according to analogy, must occur in every one of the 
minutest fibres, either does not take place at all im the organic 
fibres, or does so at a much later period, and their peculiarity 
therefore consists in their remaining stationary at an earlier 
stage of development, and either never attaining to the higher 
development of ordinary nerves, or only at a much later period, 
(a point which might be decided by comparing their numbers 
in old and young individuals.) One can conceive that the 
function of the organic nerves, whether it be actually a che- 
mico-vital one, or consist merely in the production of in- 
voluntary motion, requires less-developed nerves, in the same 
way that the involuntary muscles do not attain the same de- 
gree of development as the voluntary. 


2. Ganglion-globules. 


These occur in the gray substance of the brain and spinal 
cord and in the ganglia, having generally the appearance of 
comparatively large granulous globules, enclosing a round vesi- 
cle, placed eccentrically, and which again exhibits in its 
interior one or two small dark points. According to Remak, 
two of these vesicles sometimes occur in one globule. Valentin 
(Nov. act. Acad. Leopold. xviii, p. 196), calls attention to 
the similarity between their composition and that of the egg, 
he compares the vesicle of the ganglion-globules to the germi- 
nal vesicle, their parenchyma to the yelk-substance, and ascribes 


GANGLION-GLOBULES. 153 


a protecting investment of fibres resembling areolar tissue to 
both structures. This is certainly a very striking comparison, 
but the external investment must not in either instance be re- 
garded as a something wnessential, as a structure composed of 
other elementary parts, for the ganglion-globules, like the yelk, 
are true cells, and their external covering is an essential com- 
ponent part of them; it is the cell-membrane. The vitelline 
membrane of the bird’s egg, while contained in the ovary, 
is perfectly structureless, not composed of more minute ele- 
mentary parts ; the same is the case with the investment of the 
ganglion-globules. They are both of them true simple cells. 
The parenchyma of the ganglion-globules forms the cell-con- 
tents, and the vesicle in their interior is the cell-nucleus; the 
small corpuscles which it contains are the nucleoli. The vesicle 
of the ganglion-globules lies, as in other cells, eccentrically 
upon the internal surface of the cell-membrane. ‘This cell- 
membrane may be most distinctly observed in the ganglion- 
globules of the sympathetic nerves of the frog, previous to their 
junction with the sacral plexus. (See pl. LV, fig. 10, a.) It there 
appears comparatively dark, and sharply defined, both externally 
and internally, so that its thickness may be readily measured. 
Valentin has already remarked, that the capsule of the gan- 
glon-globules is thicker in the lower animals. In the situation 
before mentioned in the frog, it seems as though a ganglion- 
globule were sometimes formed within another cell. (See fig. 
10, 4.) The ordinary contents of these ganglion-globules is a 
minutely-granulous, yellowish substance. On one occasion, 
however, I saw a ganglion-globule from the head of an ox (I 
do not precisely know from what part it was taken), in which 
the granulous appearance was confined to the surface, the inte- 
rior being clear,—a fact which was rendered distinctly percepti- 
ble by causing the globule to roll about. It is nothing remarkable 
that two nuclei should sometimes occur in one ganglion-glo- 
bule ; we have observed this already in several cells, im those 
_of cartilage for instance. In those instances, however, only 
one of them was the true cell-nucleus, the cytoblast of the car- 
tilage-cell, the other being a subsequent formation within the 
cell. 


154 CAPILLARY VESSELS. 


3. Capillary vessels. 


Plate II, fig. 9, represents two stellate pigment-cells, which 
have coalesced at a. In that instance two cells had been gene- 
rated at some distance from one another, their bodies may still 
be distinguished as two spots somewhat thicker than the rest 
of the structure. These cells became elongated on different 
sides into hollow processes, which, like the cavities of the bodies 
of the cells, are filled with pigment. Two processes of the two 
cells came into contact at a, and then coalesced, the separation 
at the point of union appears to have been absorbed also at the 
same time, so that the cavities of the two cells communicate 
iunmediately with one another; at all events there is no appa- 
rent interruption to the pigment, which forms the contents of 
the cells and their prolongations. (See page 78.) Now, if we 
imagine several such stellate cells to be developed on a large 
surface at similar distances from one another, and the several 
prolongations issuing from each separate cell to coalesce with 
those issuing from the other cells, in the manner represented 
in the figure at a, the result will be a network of canals ex- 
tending over the entire surface, and all communicating with 
each other. The size of the meshes of the network is deter- 
mined by the distance of the cells from each other, and by the 
number of the prolongations issuing from each cell. Such, 
then, appears to be the process by which the capillary vessels 
are formed. 

The observations, on which this mode of formation of the 
capillary vessels is based, were made partly on the tails of very 
young tadpoles, and partly on the germinal membrane of the 
hen’s egg. They are as follows: 

1. The capillary vessels, in the tail both of the fully-deve- 
loped and young tadpoles, are seen to be surrounded by a 
thin, but distinctly perceptible membrane, which does not ex- 
hibit any fibrous arrangement. (See pl. IV, fig. 11.) The 
variety in the thickness of this membrane in different in- 
stances sufficiently explains why we cannot distmguish it in all 
capillary vessels, just as we cannot detect the cell-membrane 
even in the blood-corpuscles, although there can be no doubt 
of its existence. Where the capillary vessels exhibit a fibrous 


© soe Pay se 2 te la learn ipa mmc he se > 


CAPILLARY VESSELS. 155 


structure, they have arrived at a more complicated stage of 
their formation, and I regard such fibres as distinct from their 
cell-membrane. 

2. Very distinct cell-nuclei occur at different spots upon 
the walls of the capillaries, both of the young and fully-deve- 
loped tadpole. They appear to lie either in the thickness of 
the wall, or on the internal surface of the vessels, on which 
they often form a projection. (See fig. 11.) They admit of a 
double explanation. They are either the nuclei of the primary 
cells of the capillaries, or nuclei of epithelial cells, which in- 
vest the capillary vessels. It is true that epithelial cells oceur 
im vessels which have a great resemblance to capillary vessels, 
if they are not actually such, as may be very distinctly seen 
in the vessels of the membrana capsulo-papillaris in a foetal 
pig of from four to six inches long, where some of them pro- 
ject, in the form of half-spheres, into the cavity of the vessel ; 
but there were no epithelial cells perceptible surrounding the 
nuclei in the capillaries of the tadpole’s tail. On the contrary, 
these nuclei frequently seemed to le free upon the internal 
wall of the vessel, and must have been much more abundant 
had they been nuclei of epithelial cells. That these are the 
nuclei of the primary cells of the capillaries is, therefore, most 
probable, although this exclusive argument by no means decides 
the question. 

3. In the tail of very young tadpoles, the capillary network 
presents, besides the ordinary cylindrical canals which have an 
equal diameter, and in which the blood flows in a regular cur- 
rent, other vessels of an irregular form. Unfortunately I 
neglected to make a drawing of them ; they accord, however, 
in all essential particulars with the capillaries of the germinal 
membrane of the hen’s egg represented in pl. IV, fig. 12, 
except that the meshes of the vascular network are much 
larger in the tail of the tadpole. They are not regularly 
cylindrical. They are generally widest in situations where 
branches are given off, sometimes wider even than the ordi- 
nary capillary vessels. (See a, 6 in figure 12.) The branches 
diminish very rapidly as they leave those broad parts, and 
widen again as they approach another dilated portion. They 
present every degree of narrowing from vessels in which it 
could scarcely be remarked, to those which are reduced so 


156 CAPILLARY VESSELS. 


much as to be scarcely thicker than a fibre of areolar tissue 
(as inc). Branches are also sometimes given off from these 
wider parts, which likewise diminish very rapidly to the same 
degree of minuteness, without reaching another dilated part 
(as at de), and which are, therefore, blind ones. According 
to the above view of the development of the capillaries, these 
appearances may be explained in the following manner: the 
wider portions, a, b, &c., are the bodies of the primary cells. 
Hollow processes, as at d, are sent out from the bodies of the 
cells as the result of a more vigorous growth in different situ- 
ations, precisely as is the case in all stellate cells. These 
prolongations meet with similar ones from other cells, and thus 
produce the form c. But being hollow, they are capable of 
expansion during their growth, and thus the canal ¢ becomes 
converted into jf, and at length into g, which is as wide as an 
ordinary capillary vessel. A more accurate analysis of the 
observations, however, is necessary to enable us to judge of 
the correctness of this explanation. It might be doubted, in 
the first place, whether these were really capillaries. The blood 
flows uninterruptedly through the ordinary capillaries, but there 
are no blood-corpuscles in these canals, at least in the more 
minute ones; they are, therefore, more difficult to discover, 
and readily give rise to a doubt whether they are canals. 
But their direct continuity with the ordinary capillaries may be 
clearly demonstrated, and blood-corpuscles actually enter the 
wider ones. If they be true capillary vessels, they may either 
be ordinary ones in a state of contraction, or they must repre- 
sent a certain stage of their development. But if it be difficult 
to conceive that a capillary vessel can have the power to con- 
tract itself almost to the minuteness of a filament of areolar 
tissue, such an assumption cannot be supported at all in 
respect to the blind branches, which do not join any other 
vessel, as at d. This form might, indeed, be admitted to be a 
certain stage of development, although not of the kind de- 
scribed above; but branches might be sent off from the 
capillaries already existing, which again might give off others. 
The objection, that such an explanation does not account for 
the varying width of these capillaries, might be met by as- 
suming that circumstance to depend upon the surrounding 
substance. It is, therefore, necessary to see the primary cells 


_ 


ee a 


CAPILLARY VESSELS. , 157 


previous to their union with the actual capillaries. Now it 
is certain that a great many stellate cells are found in the 
tail of the tadpole. They lie beneath the epithelium and pig- 
ment-cells on the same plane with the capillary vessels; are 
smaller than the pigment-cells, and contain a colourless or 
palish yellow substance; they send off processes on different 
sides, which vary in number very much in different instances, 
but are generally short, and for the most part do not join 
with processes from other cells. Their shape has no sort of 
connexion with that of the pigment-cells which lie above them, 
for when, as is the case in many larve, the latter only send 
off prolongations on two sides, these cells exhibit several pro- 
cesses on different sides. They cannot, therefore, be young 
pigment-cells. Such branches of the capillaries, as those at d, 
sometimes appear to be connected with one of those stellate 
cells, and the others might, therefore, be regarded as young 
cells of capillary vessels which had not as yet begun to 
anastomose. These anastomoses, however, are not sufficiently 
evident to enable me positively to assert their existence. The 
great number of these stellate cells, and their presence at all ages 
of the tadpole, are also circumstances unfavorable to the suppo- 
sition that they are primary cells of capillaries. They might, 
indeed, be conceived to indicate a lower stage of development, 
as not having yet undergone any change, and that eventually 
capillary vessels may be developed from some, whilst others 
continue their existence without such a transformation, and 
fill the place of cells of areolar tissue. That, however, would 
be somewhat too hypothetical, and I shall, therefore, not ad- 
duce these cells as proof of the existence of primary cells of 
capillary vessels. The uncertainty which attaches to the ob- 
servations on this poimt in the tail of the tadpole appears, 
however, to be removed when we examine the incubated 
hen’s egg. 

4, When the germinal membrane of an hen’s egg which 
has been subjected to thirty-six hours’ incubation (at which 
period the formation of red blood has commenced, and is dis- 
tinctly perceptible), is placed under the microscope, and the 
area pellucida examined with a magnifying power of 450, the 
capillary vessels are readily distinguished in it by their yel- 


158 » CAPILLARY VESSELS. 


lowish-red colour. Notwithstanding repeated endeavours, I 
cannot succeed at this season of the year when the hens are 
moulting, in subjecting eggs to incubation for so long a period, 
I can, therefore, only give a representation of these vessels 
from a recollection of what I observed in the early part of 
this year. (See pl. IV, fig. 12.) In some situations the capil- 
laries are perfect, and connected with the larger vessels ; at 
others they have the appearance represented in the figure, and 
illustrated previously by observations on the tail of the tadpole. 
In addition to these capillaries, which form a network of 
canals of irregular caliber and give off blind branches, some 
separate irregular corpuscles are seen, such as / and 7, which 
do not appear to be connected with the vascular network. 
These bodies send off blind processes of various forms in 
different directions, and have the appearance, therefore, of 
stellate cells. They have a yellowish-red colour, like that of 
the bone-capillaries, which circumstance is alone sufficient to 
suggest the supposition that they are cells of capillary vessels 
in progress of development. This becomes much more pro- 
bable, when we observe some of these corpuscles, such as 4, 
already connected with the true capillaries. We may, there- 
fore, with a high degree of probability at least, regard them 
as the primary cells of capillary vessels; and in that case the 
description of the formation of these vessels, previously given, 
would be the correct one. The following would, therefore, be 
the mode in which the formation of the capillaries and of the 
blood takes place in the germinal membrane: among the 
cells which compose the germinal membrane, some which are 
deposited at certain distances from one another, are deve- 
loped into the primary cells of capillary vessels by becoming 
elongated on different sides so as to form stellate cells. 
The processes of the different cells come imto contact and 
coalesce, the septa are absorbed, and in this manner a network 
of canals of very irregular caliber is produced, the prolonga- 
tions of the primary cells being much thinner than the bodies 
of the cells. These processes of the cells or passages of com- 
munication undergo expansion until they and the bodies of 
the cells all attain one equal width, until, in fact, a network of 
canals of uniform caliber is formed. The fluid portion of the 


— +S a 


ree 


CAPILLARY VESSELS. 159 


blood constitutes the contents of the primary cells, as well as of 
the secondary ones—the vessels produced by their coalescence ; 
and the blood-corpuscles are young cells which are developed 
in their cavities. 


Thus this last class, comprising tissues, which, in their 
functions, are the most characteristic of the animal kingdom, 
exhibits the same principle of development that we have met 
with in all the others; namely, that cells originate in the 
first place, and that these become transformed into the ele- 
mentary parts of the tissues. The elementary cells in this 
class, however, undergo more essential changes during their 
transformation than those of any previous one. ‘They not 
only do not remain, as in the first two classes, independent, 
that is provided with a special cavity and particular wall; not 
only does a coalescence of the walls of neighbouring cells take 
place, as in the third class, but the cavities of the different 
cells also unite together in consequence of the absorption of 
the coalesced partition-walls of the several cells, so that the 
primary cells cease to exist as distinct objects. It is to a cer- 
tain extent the opposite process to that which occurred in the 
fourth ciass, where, in addition to the prolongation of the cells, 
a splitting of them into several, probably hollow, fibres, a sort 
of division of the cells took place. The type of the trans- 
formation of the primary cells, as presented by nerve, muscle, 
and capillary vessels, is not, however, altogether limited to this 
class, but has been already exhibited by previous classes, and 
even in plants. Some of the pigment-cells have been cited 
before as examples, and the generation of the cells of the liber 
observed by Meyen was brought forward as an instance of 
perfect analogy in vegetables. 

The independent existence of each separate primary cell is, 
no doubt, lost as a consequence of this perfect coalescence of 
several cells ; not so, however, its character as Cell in general. 
On the contrary, several primary cells contribute to form one 
secondary cell, having the full signification of one independent 
cell. Each secondary cell in muscle and nerve forms a closed 
Whole, and the distinction between cell-membrane and cell- 
contents or secondary deposit seems to continue throughout life. 
In this way the nerves bring every part of the body into con- 


160 CAPILLARY VESSELS. 


nexion with the central portions of the nervous system by 
means of a single uninterrupted cell. The different parts of 
the body, however, are connected together by another kind 
of uninterrupted secondary cell, namely, the capillaries. The 
capillary system, generated from several primary cells, forms 
one single secondary cell. The cavity of the secondary cell 
communicates with that of the large vessels. Researches are 
still required to decide the question whether these latter are 
mere dilatations of the capillaries, or whether they are formed 
simply by the junction of other elementary parts. In the 
latter case the capillary vessels would open into a cavity alto- 
gether distinct from their own, just as a vegetable cell opens 
into an intercellular space. It sometimes occurs that the cavi- 
ties of certain vegetable cells open directly outwards, but such 
instances are very rare. 

As a primitive muscular fasciculus, a nervous fibre and a 
capillary vessel are corresponding formations in this class; we 
may also compare these structures with the elementary parts 
of other tissues. The elementary cells of all tissues correspond 
with one another, being formed universally according to similar 
laws. A blood-corpuscle, an epithelial cell, a cartilage-cell, an 
elementary cell of areolar tissue (therefore, also a fasciculus of 
areolar tissue formed from it), correspond to an elementary 
cell of muscle, &c. There is no structure analogous to an 
entire primitive fasciculus of muscle or a secondary muscle-cell 
or a nervous fibre amongst the principal component parts of 
the tissues previously discussed, because with them the forma- 
tion of secondary cells only occurs as an exception. A mus- 
cular fasciculus differs, therefore, from a fasciculus of areolar 
tissue, and a primitive fibre of areolar tissue has no analogy 
with a primitive muscular fibre. 


. Si et +t 5 


SECTION III. 


REVIEW OF THE PREVIOUS RESEARCHES—THE FORMATIVE 
PROCESS OF CELLS——THE CELL THEORY. 


Tue two foregomg sections of this work have been devoted 
to a detailed investigation of the formation of the different 
tissues from cells, to the mode in which these cells are de- 
veloped, and to a comparison of the different cells with one 
another. We must now lay aside detail, take a more ex- 
tended view of these researches, and grasp the subject in its 
more intimate relations. The principal object of our investi- 
gation was to prove the accordance of the elementary parts of 
animals with the cells of plants. But the expression “ plant- 
like life” (pflanzen-ahnliches Leben) is so ambiguous that 
it is received as almost synonymous with growth without 
vessels; and it was, therefore, explained at page 6 that in 
order to prove this accordance, the elementary particles of 
animals and plants must be shown to be products of the same 
formative powers, because the phenomena attending their deve- 
lopment are similar ; that all elementary particles of animals 
and plants are formed upon a common principle. Having 
traced the formation of the separate tissues, we can more 
readily comprehend the object to be attained by this compa- 
rison of the different elementary particles with one another, a 
subject on which we must dwell a httle, not only because it is 
the fundamental idea of these researches, but because all 
physiological deductions depend upon a correct apprehension 
of this principle, 

When organic nature, animals and plants, is regarded as a 
Whole, in contradistinction to the inorganic kingdom, we do 
not find that all organisms and all their separate organs are 
compact masses, but that they are composed of innumerable 
small particles of a definite form. These elementary particles, 
however, are subject to the most extraordinary diversity of 

18 


162 GENERAL RETROSPECT. 


figure, especially in animals ; in plants they are, for the most 
part or exclusively, cells. This variety in the elementary 
parts seemed to hold some relation to their more diversified 
physiological function in animals, so that it might be established 
as a principle, that every diversity in the physiological signi- 
fication of an organ requires a difference in its elementary 
particles ; and, on the contrary, the similarity of two elemen- 
tary particles seemed to justify the conclusion that they were 
physiologically similar. It was natural that among the very 
different forms presented by the elementary particles, there 
should be some more or less alike, and that they might be 
divided, according to their similarity of figure, into fibres, which 
compose the great mass of the bodies of animals, into cells, 
tubes, globules, &e. The division was, of course, only one of 
natural history, not expressive of any physiological idea, and 
just as a primitive muscular fibre, for example, might seem to 
differ from one of areolar tissue, or all fibres from cells, so would 
there be in like manner a difference, however gradually 
marked between the different kinds of cells. It seemed as if 
the organism arranged the molecules in the definite forms 
exhibited by its different elementary particles, in the way 
required by its physiological function. It might be ex- 
pected that there would be a definite mode of development 
for each separate kind of elementary structure, and that it 
would be similar in those structures which were physiologi- 
cally identical, and such a mode of development was, in- 
deed, already more or less perfectly known with regard to 
muscular fibres, blood-corpuscles, the ovum (see the Supple- 
ment), and epithelium-cells. The only process common to 
all of them, however, seemed to be the expansion of their 
elementary particles after they had once assumed their proper 
form. The manner in which their different elementary par- 
ticles were first formed appeared to vary very much. In 
muscular fibres they were globules, which were placed together 
im rows, and coalesced to form a fibre, whose growth proceeded 
in the direction of its length. In the blood-corpuscles it was 
a globule, around which a vesicle was formed, and continued 
to grow; in the case of the ovum, it was a globule, around 
which a vesicle was developed and continued to grow, and 
around his again a second vesicle was formed. 


GENERAL RETROSPECT. 163 


The formative process of the cells of plants was clearly 
explained by the researches of Schleiden, and appeared to be 
the same in all vegetable cells. So that when plants were 
regarded as something special, as quite distinct from the 
animal kingdom, one universal principle of development was 
observed in all the elementary particles of the vegetable or- 
ganism, and physiological deductions might be drawn from it 
with regard to the independent vitality of the individual cells 
of plants, &e. But when the elementary particles of animals 
and plants were considered from a common point, the vege- 
table cells seemed to be merely a separate species, co-ordinate 
with the different species of animal cells, just as the entire 
class of cells was cv-ordinate with the fibres, &c., and the 
uniform principle of development in vegetable cells might be 
explained by the slight physiological difference of their elemen- 
tary particles. 

The object, then, of the present investigation was to show, 
that the mode in which the molecules composing the elemen- 
tary particles of organisms are combined does not vary 
according to the physiological signification of those particles, 
but that they are everywhere arranged according to the same 
laws ; so that whether a muscular fibre, a nerve-tube, an ovum, 
or a blood-corpuscle is to be formed, a corpuscle of a certain 
form, subject only to some modifications, a cell-nucleus, is uni- 
versally generated in the first mstance; around this corpuscle 
a cell is developed, and it is the changes which one or more 
of these cells undergo that determine the subsequent forms of 
the elementary particles ; in short, that there is one common 
principle of development for all the elementary particles of 
organisms. 

In order to establish this point it was necessary to trace 
the progress of development in two given elementary parts, 
physiologically dissimilar, and to compare them with one 
another. If these not only completely agreed in growth, 
but in their mode of generation also, the principle was 
established that elementary parts, quite distinct im a_phy- 
siological sense, may be developed according to the same laws. 
This was the theme of the first section of this work. The 
course of development of the cells of cartilage and of the 


164 GENERAL RETROSPECT. 


cells of the chorda dorsalis was compared with that of vege- 
table cells. Were the cells of plants developed merely as 
infinitely minute vesicles which progressively expand, were 
the circumstances of their development less characteristic 
than those pointed out by Schleiden, a comparison, in the 
sense here required, would scarcely have been possible. We 
endeavoured to prove in the first section that the complicated 
process of development in the cells of plants recurs in those 
of cartilage and of the chorda dorsalis. We remarked the 
similarity in the formation of the cell-nucleus, and of its 
nucleolus in all its modifications, with the nucleus of vegetable 
cells, the pre-existence of the cell-nucleus and the development 
of the cell around it, the similar situation of the nucleus in 
relation to the cell, the growth of the cells, and the thickening 
of their wall during growth, the formation of cells within 
cells, and the transformation of the cell-contents just as in 
the cells of plants. Here, then, was a complete accordance 
in every known stage in the progress of development of two 
elementary parts which are quite distinct, in a physiological 
sense, and it was established that the principle of develop- 
ment in two such parts may be the same, and so far as could 
be ascertained in the cases here compared, it is really the 
same. 

But regarding the subject from this point of view we are 
compelled to prove the universality of this principle of develop- 
ment, and such was the object of the second section. For so 
long as we admit that there are elementary parts which originate 
according to entirely different laws, and between which and 
the cells which have just been compared as to the principle of 
their development there is no connexion, we must presume 
that there may still be some unknown difference in the laws 
of the formation of the parts just compared, even though 
they agree in many points. But, on the contrary, the greater 
the number of physiologically different elementary parts, which, 
so far as can be known, originate in a similar manner, and 
the greater the difference of these parts in form and physio- 
logical signification, while they agree in the perceptible phe- 
nomena of their mode of formation, the more safely may 
we assume that all elementary parts have one and the same 


GENERAL RETROSPECT. 165 


fundamental principle of development. It was, in fact, 
shown that the elementary parts of most tissues, when 
traced backwards from their state of complete development 
to their primary condition are only developments of cells, 
which so far as our observations, still incomplete, extend, 
seemed to be formed in a similar manner to the cells com- 
pared in the first section. As might be expected, according 
to this principle the cells, in their earliest stage, were almost 
always furnished with the characteristic nuclei, in some the 
pre-existence of this nucleus, and the formation of the cell 
around it was proved, and it was then that the cells began to 
undergo the various modifications, from which the diverse forms 
of the elementary parts of animals resulted. Thus the apparent 
difference in the mode of development of muscular fibres and 
blood-corpuscles, the former originating by the arrangement of 
globules in rows, the latter by the formation of a vesicle 
around a globule, was reconciled in the fact that muscular 
fibres are not elementary parts co-ordinate with blood-cor- 
puscles, but that the globules composimg muscular fibres at 
first correspond to the blood-corpuscles, and are like them, 
vesicles or cells, containing the characteristic cell-nucleus, 
which, like the nucleus of the blood-corpuscles, is probably 
formed before the cell. The elementary parts of all tissues 
are formed of cells in an analogous, though very diversified 
manner, so that it may be asserted, that there is one universal 
principle of development for the elementary parts of organisms, 
however different, and that this principle is the formation of 
cells. Thisis the chief result of the foregoing observations. 

The same process of development and transformation of 
cells within a structureless substance is repeated in the for- 
mation of all the organs of an organism, as well as in the 
formation of new organisms ; and the fundamental phenomenon 
attending the exertion of productive power in organic nature 
is accordingly as follows: a structureless substance is pre- 
sent in the first instance, which lies either around or in the inte- 
rior of cells already existing ; and cells are formed wm it in ac- 
cordance with certain laws, which cells become developed in 
various ways into the elementary parts of organisms. 

The development of the proposition, that there exists one gene- 


166 GENERAL RETROSPECT. 


ral principle for the formation of all organic productions, and 
that this principle is the formation of cells, as well as the conclu- 
sions which may be drawn from this proposition, may be com- 
prised under the term cell-theory, using it m its more extended 
signification, whilst in a more limited sense, by theory of the 
cells we understand whatever may be inferred from this pro- 
position with respect tothe powers from which these pheno- 
mena result. 

But though this principle, regarded as the direct result of 
these more or less complete observations, may be stated to be 
generally correct, it must not be concealed that there are some 
exceptions, or at least differences, which as yet remain unex- 
plained. Such, for instance, is the splitting mto fibres of the 
walls of the cells in the interior of the chorda dorsalis of osseous 
fishes, which was alluded to at page 14. Several observers 
have also drawn attention to the fibrous structure of the firm 
substance of some cartilages. In the costal cartilages of old 
persons for example, these fibres are very distinct. They do 
not, however, seem to be uniformly diffused throughout the carti- 
lage, but to be scattered merely here and there. I have not ob- 
served them at all in new-born children. It appears as if the 
previously structureless cytoblastema in this instance became 
split into fibres; I have not, however, investigated the point 
accurately. Our observations also fail to supply us with any 
explanation of the formation of the medullary canaliculi in 
bones, and an analogy between their mode of origin and that 
of capillary vessels, was merely suggested hypothetically. The 
formation of bony lamelle around these canaliculi, is also an 
instance of the cytoblastema assuming a distinct form. But 
we will return presently to an explanation of this phenomenon 
that is not altogether improbable. In many glands, as for 
instance, the kidneys of a young mammalian fcetus, the 
stratum of cells surrounding the cavity of the duct, is enclosed 
by an exceedingly delicate membrane, which appears to be an 
elementary structure, and not to be composed of areolar tissue. 
The origin of this membrane is not at all clear, although we 
may imagine various ways of reconciling it with the formative 
process of cells. (These gland-cylinders seem at first to have 
no free cavity, but to be quite filled with cells. In the kidneys 


GENERAL RETROSPECT. 167 


of the embryos of pigs, I found many cells in the cylinders, 
which were so large as to occupy almost the entire thickness 
of the canal. In other cylinders, the cellular layer, which 
was subsequently to line their walls, was formed, but the cavity 
was filled with very pale transparent cells, which could be 
pressed out from the free end of the tube.) 

These and similar phenomena may remain for a time un- 
explained. Although they merit the greatest attention and re- 
quire further investigations, we may be allowed to leave 
them for a moment, for history shows that in the laying down 
of every general principle, there are almost always anomalies 
at first, which are subsequently cleared up. 

The elementary particles of organisms, then, no longer le 
side by side unconnectedly, ike productions which are merely 
capable of classification in natural history, according to simi- 
larity of form; they are united by a common bond, the 
similarity of their formative principle, and they may be com- 
pared together and physiologically arranged in accordance 
with the various modifications under which that principle is 
exhibited. In the foregoing part of this work, we have treated 
of the tissues im accordance with this physiological arrange- 
ment, and have compared the different tissues with one 
another, proving thereby, that although different, but similarly 
formed, elementary parts may be grouped together in a natural- 
history arrangement, yet such a classification does not neces- 
sarily admit of a conclusion with regard to their physiological 
position, as based upon the laws of development. Thus, for 
example, the natural-history division, “ cells,’ would, in a 
general sense, become a physiological arrangement also, inas- 
much as most of the elementary parts comprised under it have 
the same principle of development ; but yet it was necessary to 
separate some from this division ; as, for instance, the germi- 
nal vesicle, all hollow cell-nuclei, and cells with walls composed 
of other elementary parts, although the germinal vesicle is a 
cell in the natural-history sense of the term. It does not 
correspond to an epithelium-cell, but to the nucleus of one. 
The difference in the two modes of classification was still 
more remarkable in respect to fibres. The mode of their 
origin is most varied, for, as we saw, a fibre of areolar tissue 


168 SURVEY OF CELL-LIFE. 


is essentially different from a muscular fibre; while, on the 
other hand, a whole primitive muscular fasciculus is identical 
in its mode of origin with a nervous fibre, and so on. ‘The 
existence of a common principle of development for all the 
elementary parts of organic bodies lays the foundation of a 
new section of general anatomy, to which the term philoso- 
phical might be applied, having for its object—firstly, to 
prove the general laws by which the elementary parts of 
organisms are developed; and, secondly, to poimt out the dif- 
ferent elementary parts in accordance with the general princi- 
ple of development, and to compare them with one another. 


SURVEY OF CELL-LIFE. 


The foregomg investigation has conducted us to the princi- 
ple upon which the elementary parts of organized bodies are 
developed, by tracing these elementary parts, from their per- 
fected condition, back to the earlier stages of development. 
Starting now from the principle of development, we will recon- 
struct the elementary parts as they appear in the matured 
state, so that we may be enabled to take a comprehensive view 
of the laws which regulate the formation of the elementary 
particles. We have, therefore, to consider—1, the cytoblas- 
tema; 2, the laws by which new cells are generated in the 
cytoblastema ; 3, the formative process of the cells themselves ; 
4, the very various modes in which cells are developed into the 
elementary parts of organisms. 


Cytoblastema.—The cytoblastema, or the amorphous sub- 
stance in which new cells are to be formed, is found either 
contained within cells already existing, or else between them in 
the form of intercellular substance. The cytoblastema, which 
hes on the outside of existing cells, is the only form of 
which we have to treat at present, as the cell-contents form 
matter for subsequent consideration. Its quantity varies ex- 
ceedingly, sometimes there is so little that it cannot be recog- 
nized with certainty between the fully-developed cells, and can 
only be observed between those most recently formed ; for 
instance, in the second class of tissues ; at other times there is 


SURVEY OF CELL-LIFE. 169 


so large a quantity present, that the cells contained in it do 
not come into contact, as is the case in most cartilages. The 
chemical and physical properties of the cytoblastema are not 
the same in all parts. In cartilages it is very consistent, and 
ranks among the most solid parts of the body; in areolar 
tissue it is gelatinous; in blood quite fluid. These physical 
distinctions imply also a chemical difference. The cytoblas- 
tema of cartilage becomes converted by boiling into gelatine, 
which is not the case with the blood ; and the mucus in which 
the mucus-cells are formed differs from the cytoblastema of 
the cells of blood and cartilage. The cytoblastema, external 
to the existing cells, appears to be subject to the same 
changes as the cell-contents ; in general it is a homogeneous 
substance ; yet it may become minutely granulous as the re- 
sult of a chemical transformation, for instance, in areolar 
tissue and the cells of the shaft of the feather, &c. As a 
general rule, it diminishes in quantity, relatively with the deve- 
lopment of the cells, though it seems that in cartilages there 
may be even a relative increase of the cytoblastema propor- 
tionate to the growth of the tissue. The physiological relation 
which the cytoblastema holds to the cells may be twofold: 
first, it must contain the material for the nutrition of the 
cells ; secondly, it must contain at least a part of what remains 
of this nutritive material after the cells have withdrawn from 
it what they required for their growth. In animals, the cyto- 
blastema receives the fresh nutritive material from the blood- 
vessels ; in plants it passes chiefly through the elongated cells 
and vascular fasciculi; there are, however, many plants which 
consist of simple cells, so that there must also be a transmis- 
sion of nutrient fluid through the simple cells ; blood-vessels and 
vascular fasciculi are, however, merely modifications of cells. 


Laws of the generation of new cells in the cytoblastema.— 
In every tissue, composed of a definite kind of cells, new cells 
of the same kind are formed at those parts only where the 
fresh nutrient material immediately penetrates the tissue. 
On this depends the distinction between organized or vas- 
cular, and unorganized or non-vascular tissues. In the former, 
the nutritive fluid, the liquor sanguinis, permeates by means 
of the vessels the whole tissue, and therefore new cells origi- 


170 SURVEY OF CELL-LIFE. 


nate throughout its entire thickness. Non-vascular tissues,. 


on the contrary, such as the epidermis, receive the nutri- 
tive fluid only from the tissue beneath; and new cells 
therefore originate only on their under surface, that is, at the 
part where the tissue is in connexion with organized sub- 
stance. So also in the earlier period of the growth of carti- 
lage, while it is yet without vessels new cartilage-cells are 
formed around its surface only, or at least in the neigh- 
bourhood of it, because the cartilage is connected with 
the organized substance at that part, and the cytoblastema 
penetrates from without. We can readily conceive this to be 
the case, if we assume that a more concentrated cytoblastema 
is requisite for the formation of new cells than for the growth 
of those already formed. In the epidermis, for instance, the 
cytoblastema below must contain a more concentrated nutri- 
tive material. When young cells are formed in that situation, 
the cytoblastema, which penetrates into the upper layers, is less 
concentrated, and may therefore serve very well for the growth 
of cells already formed, but not be capable of generating 
new ones. ‘This constitutes the distinction which was formerly 
made between a growth by apposition and one by intussuscep- 
tion; “ growth by apposition” is a correct term, if it be 
applied to the generation of new cells, and not to the growth 
of those already existing, the new cells in the epidermis for 
example, are formed only on its under surface, and are pushed 
upwards when other new ones are formed beneath them; 
but the new cells are generated throughout the entire thick- 
ness of the organized tissues. The cells, however, grow in- 
dividually by intussusception in both instances. The bones oc- 
cupy,to a certain extent,a middle position between the organized 
and unorganized tissues. The cartilage in the first mstance 
has no vessels, and the new cells are, therefore, formed in the 
neighbourhood of the external surface only ; at a subsequent 
period it receives vessels, which traverse the medullary or Haver- 
sian canals, the latter, however, are not sufficiently numerous to 
allow of the entire tissue becoming equably saturated with the 
fluid parts of the blood, a process which would he still further 
impeded by the greater firmness of cartilage and bone. 
According to the above law, then, the formation of new 
cytoblastema and new cells may take place partly upon the 


SURVEY OF CELL-LIFE. 171 


surface and partly around these medullary canals. Now, the 
structure of bone becomes most simple, if we assume that, 
in consequence of the firmness of the osseous substance, this 
process goes on in layers, which do not completely coalesce 
together. It must consist of a double system of layers, one 
being concentric to each of the medullary canals, and the 
other to the external surface of the bone. When the bone is 
hollow, the layers must also be concentric to the cavity; and 
when small medullary cavities exist in the place of canals, 
as in the spongy bones, the layers must also be concentric to 
them. The difference in the growth of animals and plants 
also rests upon the same law. In plants, the nutritive fluid 
is not so equably distributed throughout the entire tissues, 
as it is in the organized tissues of animals, but is conveyed in 
isolated fasciculi of vessels, widely separated from one another, 
more after the manner of bone. These fasciculi of vessels are 
also observed to be surrounded with small (most likely 
younger) cells, so that, in all probability, the formation of 
their new cells also takes place around these vessels, as it does 
im bones around the medullary canaliculi. In the stem of 
dicotyledonous plants the sap is conducted hetween the bark 
and the wood, and on that account the new cells are generated 
in strata concentric to the layers of the previous year. The 
variety in the mode of growth, as to whether the new cells 
are developed merely in separate situations in the tissue, or 
equally throughout its whole thickness, does not, therefore, 
constitute any primary distinction, but is the consequence of 
a difference in the mode in which their nutritive fluid is 
conveyed. 

The generation of cells of a different character, such as fat- 
cells, in the interior of a non-vascular tissue (in cartilage 
which does not as yet contain vessels, for example), appears at 
first sight to form an exception to the law just laid down. But 
such is not really the case; the circumstance is capable of 
two explanations, either the cytoblastema for this kind of 
cells is furnished by the true cells of the tissue only when they 
have attained a certain stage of their development, or, the 
cytoblastema which penetrates into the depth of the tissue 
contains the nutritive material for the true cells of the 
tissue in a less concentrated state, whilst it is still sufficiently 


172 SURVEY OF CELL-LIFE. 


impregnated with the nutritive material for the other kind of 
cells. 

According to Schleiden, new cells are never formed in the 
intercellular substance in plants ; in animals, on the contrary, 
a generation of cells within cells is the less frequent mode, but 
this does occur, and in such a way, that a threefold or four- 
fold generation may take place im succession within one cell. 
Thus, according to R. Wagner’s observations (see the Supple- 
ment), the Graafian vesicle appears to be an elementary cell ; 
the ovum is developed within it in ike manner as an element- 
ary cell; within this, again, according at least to observations 
made upon the bird’s egg, cells are generated, some of which 
contain young cells. It appears also, that a formation of 
true cartilage-cells can sometimes take place within those 
which already exist, and that young cells (fat-cells?) may 
be generated within them agai. Several such examples 
might be brought forward; but by far the greater portion 
of the cells of cartilage are formed in the cytoblastema on 
the outside of the cells already present, and we never meet 
with a generation of cells within cells in the case of fibre, 
muscle, or nerve. 


General phenomena of the formation of cells. Round 
corpuscles make their appearance after a certain time in the 
cytoblastema which, in the first instance, is structure- 
less or minutely granulous. These bodies may either be 
cells in their earliest condition (and some may be recognized 
even at this stage), that is, hollow vesicles furnished with a 
peculiar structureless wall, cells without nuclei, or they may 
be cell-nuclei or the rudiments of cell-nuclei, round which cells 
will afterwards be formed. 

The cells without nuclei, or, more correctly, the cells in 
which no nuclei have as yet been observed, occur only 
in the lower plants, and are also rare in animals, For the 
present, however, the followmg must be regarded as such, 
viz.: the young cells contained within others in the chorda 
dorsalis (see p. 13), the cells of the yelk-substance in the 
bird’s egg (p. 50), the cells in the mucous layer of the ger- 
minal membrane of the bird’s egg (p. 60), and some cells of 
the crystalline lens (p.88). Pl. I, fig. 10, c¢, represents one 


— 


a 


SURVEY OF CELL-LIFE. 173 


of these cells without nuclei. Thus the mode of growth, in 
this instance, is similar to that of the nucleated cells, after the 
formation of their cell-membrane 

By far the greater portion of the animal body, at least 
ninety-nine hundredths of all the elementary parts of the bodies 
of mammalia are developed from nucleated cells. 


The cell-nucleus is a corpuscle, having a very characteristic 
form, by which it may in general be easily recognized. It 
is rather round or oval, spherical or flat. In the majority of fully- 
developed animal cells its average size would be about 0:0020- 
0:0030 Paris inch; but we meet with nuclei which are very 
much larger, and others, again, much smaller than this. The 
germinal vesicle of the bird’s egg may be regarded as the 
largest cell-nucleus; the nuclei of the blood-corpuscles of 
warm-blooded animals afford examples of very small cell- 
nuclei. Ifthe latter were but a very little smaller they would 
escape observation altogether, and the blood-corpuscles would 
then appear to be cells without nuclei. No other structure 
can be detected in these very small nuclei, nor can their cha- 
racteristic form be further demonstrated. On the other hand, 
that of the larger blood-corpuscles may be distinctly recog- 
nized as a cell-nucleus. 

The cell-nucleus is generally dark, granulous, often some- 
what yellowish ; but some occur which are quite pellucid and 
smooth. It is either solid, and composed of a more or less 
minutely granulated mass, or hollow. Most nuclei of animal 
cells exhibit more or less distinct trace of a cavity, at least, 
their external contour is generally somewhat darker, and the 
substance of the nucleus seems to be somewhat more com- 
pact at the circumference. The nucleus may often be traced 
through its progressive stages of development from a solid 
body to a perfect vesicle ; this may be observed in the nuclei 
of the cartilage-cells in the branchial cartilages of tadpoles. 
The membrane of the cell-nucleus and its contents may be 
distinguished in those which are hollow. The membrane is 
smooth, structureless, and never of any remarkable thickness, 
that of the germinal vesicle being the thickest. The con- 
tents are either very minutely granulous, especially in the 
small hollow cell-nuclei, or pellucid, as in the germinal 
vesicle, and the larger nuclei in the cells of the branchial carti- 


174 SURVEY OF CELL-LIFE. 


lages of the tadpole, or larger corpuscles may be subsequently 
formed in the interior of hollow nuclei, for instance, the 
innumerable corpuscles in the germinal vesicle of the fish, 
and fat-globules in the nucleus of the fat-cells m the cranial 
cavity of fishes. 

The nucleus, in most instances, contains one or two, more 
rarely three or four small dark corpuscles, the nucleoh. Their 
size varies from that of a spot which is scarcely discernible to 
that of Wagner’s spot (macula germinativa) in the germinal vesi- 
cle. Nucleoli cannot be distinctly recognized in all cell-nuclei. 
They may be distinguished from the larger corpuscles, which are 
sometimes developed in certain hollow nuclei, from the cireum- 
stance of their being formed at a much earlier period; they 
exist, indeed, before the cell-nucleus. They are placed eccen- 
trically in the round nuclei, and in the hollow ones are dis- 
tinctly seen to lie upon the internal surface of the wall. It is 
very difficult to ascertain their nature; it may also vary very 
much in different cells, They sometimes appear to be capable 
of considerable enlargement, as in the nuclei of the fat-cells in 
the cranial cavity of the fish, and in such instances often have 
the appearance of fat. According to Schleiden, hollow nucleoli 
also frequently occur in plants. 

Most cell-nuclei agree in the peculiarity of not being dis- 
solved, or rendered transparent by acetic acid, at least not 
rapidly so, whilst the cell-membrane of animal cells is in 
most cases very sensitive to its action. Some cells, (such 
as those of the yelk-cavity of the egg, plate II, fig. 3,) 
which have no perceptible nucleus of the ordinary form, ex- 
hibit a globule having the appearance of a fat-globule, which 
grows as the cell expands, though not in the same proportion, 
and was probably formed previous to the cell. Whether such 
a globule have the signification of a nucleus or not, must re- 
main an undecided question. 


The formation of the cell-nucleus. In plants, according to 
Schleiden, the nucleolus is first formed, and the nucleus around 
it. The same appears to be the case in animals. According 
to the observations of R. Wagner on the development of ova 
in the ovary of Agrion virgo,' the germinal spot is first 


' See Wagner, Beitraige zur Geschichte der Zeugung und Entwickelung; Erster 
Beitrag., tab. II, fig. 1. 


SURVEY OF CELL-LIFE. 175 


formed, and around that the germinal vesicle, which is the 
nucleus of the ovum-cell, Eizelle.'| The youngest germinal 
vesicle there represented by Wagner, appears to be hollow. 
This is not generally the case, however, in the formation of 
cell-nuclei. Plate III, fig. 1, e, appears to be a cell-nucleus 
of a cartilage-cell in the act of forming. A small round 
corpuscle is there seen, surrounded by some minutely gra- 
nulous substance, whilst the rest of the cytoblastema is 
homogeneous. This granulous substance is gradually lost 
around the object; at a subsequent period it begins to 
be sharply defined, and then exhibits the form of a cell- 
nucleus, which continues to grow for a certain period. (See 
pl. III, fig. 1, a, 6.) Such a nucleus usually appears solid 
in the first imstance, and many nuclei remain in this con- 
dition; in others, on the contrary, the portion of the sub- 
stance situated nearest to the external surface continually 
becomes darker, and not unfrequently at last forms a dis- 
tinctly perceptible membrane, so that the nucleus is hollow 
in such instances. The formative process of the nucleus 
may, accordingly, be conceived to be as follows: A nucle- 
olus is first formed; around this a stratum of substance 
is deposited, which is usually minutely granulous, but not as 
yet sharply defined on the outside. As new molecules are 
constantly being deposited in this stratum between those 
already present, and as this takes place within a precise dis- 
tance of the nucleolus only, the stratum becomes defined 
externally, and a cell-nucleus having a more or less sharp con- 
tour is formed. The nucleus grows by a continuous depo- 
sition of new molecules between those already existing, that 
is, by intussusception. If this go on equably throughout the 
entire thickness of the stratum, the nucleus may remain solid ; 
but if it go on more vigorously in the external part, the latter 
will become more dense, and may become hardened into 
a membrane, and such are the hollow nuclei. The circum- 
stance of the layer generally becoming more dense on its 
exterior, may be explained by the fact that the nutritive fluid 
is conveyed to it from the outside, and is therefore more con- 
centrated in that situation. Now if the deposition of the new 


' See the Supplement. 


176 SURVEY OF CELL-LIFE. 


molecules between the particles of this membrane takes place 
in such a manner that more molecules are deposited between 
those particles which le side by side upon its surface than 
there are between those which lie one beneath another in its 
thickness, the expansion of the membrane must proceed more 
vigorously than its increase in thickness, and therefore a con- 
stantly imereasing space must be formed between it and the 
nucleolus, whereby the latter remains adherent to one side of 
its internal surface. 

I have made no observations on the formation of nuclei with 
more than one nucleolus. But it is easy to comprehend 
how it may occur, if we conceive that two nucleoli may le 
so close together that the layers which form around them 
become united before they are defined externally, and that by 
the progressive deposition of new molecules, the external limi- 
tation is so effected that two corpuscles are enclosed by it at 
the same time, and then the development proceeds as though 
only one nucleolus were present. 

When the nucleus has reached a certain stage of develop- 
ment, the cell is formed around it. The following appears to 
be the process by which this takes place. A stratum of sub- 
stance, which differs from the cytoblastema, is deposited upon 
the exterior of the nucleus. (See pl. III, fig. 1, d.) In the 
first instance this stratum is not sharply defined externally, 
but becomes so in consequence of the progressive deposition 
of new molecules. The stratum is more or less thick, some- 
times homogeneous, sometimes granulous ; the latter is most fre- 
quently the case in the thick strata which occur in the forma- 
tion of the majority of animal cells. We cannot at this period 
distinguish a cell-cavity and cell-wall. The deposition of new 
molecules between those already existing proceeds, however, 
and is so effected that when the stratum is thin, the entire 
layer—and when it is thick, only the external portion—he- 
comes gradually consolidated into a membrane. ‘The external 
portion of the layer may begin to become consolidated soon 
after it is defined on the outside; but, generally, the membrane 
does not become perceptible until a later period, when it is 
thicker and more defined internally ; many cells, however, do 
not exhibit any appearance of the formation of a cell-mem- 
brane, but they seem to be solid, and all that can be remarked 


SURVEY OF CELL-LIFE. Lae 


is that the external portion of the layer is somewhat more 
compact. 

Immediately that the cell-membrane has become consoli- 
dated, its expansion proceeds as the result of the progressive 
reception of new molecules between the existing ones, that is 
to say, by virtue of a growth by intussusception, while at the 
same time it becomes separated from the cell-nucleus. We 
may therefore conclude that the deposition of the new mole- 
cules takes place more vigorously between those which lie side 
by side upon the surface of the membrane, than it does between 
those which lie one upon another in its thickness. The inter- 
space between the cell-membrane and cell-nucleus is at the 
same time filled with fluid, and this constitutes the cell-con- 
tents. During this expansion the nucleus remains attached 
to a spot on the internal surface of the cell-membrane. If the 
entire stratum, in which the formation of the cell commenced, 
have become consolidated into a cell-membrane, the nucleus 
must lie free upon the cell-wall; but if only the external por- 
tion of the stratum have become consolidated, the nucleus must 
remain surrounded by the internal part, and adherent to a spot 
upon the internal surface of the cell-membrane. It would seem 
that the portion of the stratum which remains may be disposed 
of in two ways: either it is dissolved and forms a part of the 
cell-contents, in which case the nucleus will le free upon the 
cell-wall as before; or it gradually becomes condensed into a 
substance similar to the cell-membrane, and then the nucleus 
appears to lie in the thickness of the cell-wall. This explains 
the variety in the position of the nucleus with respect to the 
cell-membrane. According to Schleiden, it sometimes lies in 
the thickness of the membrane in plants, so that its internal 
surface, which is directed towards the cell-cavity, is covered 
by a lamella of the cell-wall. In animals it also sometimes 
appears to be slightly sunk in the cell-membrane; but I have 
never observed a lamella passing over its inner surface ; on the 
contrary, in almost all instances it les quite free, adherent 
only to the internal surface of the cell-membrane. 

The particular stage of development of the nucleus at which 
the cell commences to be formed around it varies very much. 
In some instances the nucleus has already become a distinct 

12 


178 SURVEY OF CELL-LIFE. 


vesicle ere it occurs; the germinal vesicle, for example; in 
others, and this is the most common, the nucleus is still solid, 
and its development into a vesicle does not take place until a 
later period, or perhaps the change never occurs at all. When 
the cell is developed, the nucleus either remains stationary at 
its previous stage of development, or its growth proceeds, but 
not in proportion to the expansion of the cell, so that the 
intermediate space between it and the cell-membrane, the cell- 
cavity, is also constantly becoming relatively larger. If the 
growth of a cell is impeded by the neighbouring cells, or if 
the new molecules added between the existing particles of 
the cell-membrane are applied to the thickening of the 
cell-wall instead of to its expansion, it may occur that the 
nucleus becomes more vigorously expanded than the cell, and 
gradually fills a larger portion of the cell-cavity. An example 
of this was brought forward at page 23, from the branchial 
cartilages of the tadpole; on the whole, however, such instances 
are very rare. As the nuclei, in the course of their develop- 
ment, and especially in such instances as that just mentioned, 
continually lose their granulous contents and become pellucid, 
and as in some cases, the germinal vesicle for example, other 
corpuscles, such as fat-globules, &c., may be developed in these 
contents of the nucleus (a circumstance which never occurs 
with respect to the cell-cavities) it is often difficult to distin- 
guish such enlarged nuclei from young cells. The presence 
of two nucleoli is often sufficient to enable us to distinguish 
such an enlarged hollow nucleus. The observation of the 
stages of transition, between the characteristic form of the 
cell-nucleus and these nuclei which so much resemble cells, 
will also aid us in obtaining the information desired. As in 
the case of the germinal vesicle, however, a positive decision 
can only be obtained by demonstrating that such a nucleus 
has precisely the same relation to the cell that an ordinary 
cell-nucleus has; that is to say, that such a nucleus is formed 
before the cell, that the latter is formed as a stratum around 
it, and that the nucleus is afterwards surrounded by the cell. 
Whether the nucleus undergoes any further development, as 
the expansion of the cell proceeds, or not, the usual result is 
that it becomes absorbed. ‘This does not take place, however, 


SURVEY OF CELL-LIFE. | 179 


in all cases, for, according to Schleiden, it remains persistent 
in most cells in the Euphorbiacez, and the blood-corpuscles 
may be quoted as an example to the same effect in animals. 
The fact that many nuclei are developed into hollow vesicles, 
and the difficulty of distinguishing some of these hollow nuclei 
from cells, forms quite sufficient ground for the supposition 
that a nucleus does not differ essentially from a cell; that an 
ordinary nucleated cell is nothing more than a cell formed 
around the outside of another cell, the nucleus; and _ that 
the only difference between the two consists in the inner 
one being more slowly and less completely developed, after 
the external one has been formed around it. If this descrip- 
tion were correct, we might express ourselves with more pre- 
cision, and designate the nuclei as cells of the first order, and 
the ordinary nucleated cells as cells of the second order. 
Hitherto we have decidedly maintained a distinction between 
cell and nucleus; and it was convenient to do so as long as 
we were engaged in merely describing the observations. There 
can be no doubt that the nuclei correspond to one another in 
all cells; but the designation, “cells of the first order,” in- 
cludes a theoretical view of the matter which has yet to be 
proved, namely, the identity of the formative process of the 
cell and the nucleus. This identity, however, is of the greatest 
importance for our theory, and we must therefore compare 
the two processes somewhat more closely. The formation 
of the cell commenced with the deposition of a precipitate 
around the nucleus; the same occurs in the formation of the 
nucleus around the nucleolus. The deposit becomes defined 
externally into a solid stratum: the same takes place in the 
formation of the nucleus. The development proceeds no 
farther in many nuclei, and we also meet with cells which 
remain stationary at the same poit. The further development 
of the cells is manifested either by the entire stratum, or only 
the external part of it becoming consolidated into a membrane; 
this is precisely what occurs with the nuclei which undergo 
further development. The cell-membrane inereases in its 
superficies, and often in thickness also, and separates from 
the nucleus, which remains lying on the wall; the membrane 
of the hollow cell-nuclei grows in the same manner, and the 
nucleolus remains adherent to a spot upon the wall. <A trans- 


180 SURVEY OF CELL-LIFE. 


formation of the cell-contents frequently follows, giving rise to 
a formation of new products in the cell-cavity. In most of 
the hollow cell-nuclei, the contents become paler, less granu- 
lous, and in some of them fat-globules, &c., are formed. (See 
pages 173, 4.) We may therefore say that the formation of cells 
is but a repetition around the nucleus of the same process by 
which the nucleus was formed around the nucleolus, the only 
difference being that the process is more intense and complete 
in the formation of cells than in that of nuclei. 

According to the foregoing, then, the whole process of the 
formation of a cell consists in this, that a small corpuscle (the 
nucleolus) is the earliest formation, that a stratum (the nucleus) 
is first deposited around it, and then subsequently a second 
stratum (substance of the cell) around this again. The sepa- 
rate strata grow by the reception of new molecules between 
the existing ones, by intussusception, and we have here an illus- 
tration of the law, in deference to which the deposition takes 
place more vigorously in the external part of each stratum than 
it does in the internal, and more vigorously in the entire ex- 
ternal stratum than in the internal. In obedience to this law 
it often happens that only the external part of each stratum 
becomes condensed into a membrane (membrane of the nucleus 
and membrane of the cell), and the external stratum becomes 
more perfectly developed to form a cell, than the nucleus does. 
When the nucleoli are hollow, which, according to Schleiden, 
is the case in some instances in plants, perhaps a threefold 
process of the kind takes place, so that the cell-membrane 
forms the third, the nucleus the second, and the nucleolus the 
first stratum. Probably merely a single stratum is formed 
around an immeasurably small corpuscle in the case of those 
cells which have no nuclei. 


Varieties in the development of the cells in different tissues. 
Although, as we have just seen, the formative process of the 
cells is essentially the same throughout, and dependent upon a 
formation of one or many strata, and upon a growth of those 
strata by mtussusception, the changes, on the other hand, which 
the cells, when once formed, undergo in the different tissues, 
are, in their phenomena at least, much more varied. They 
may be arranged in two classes according as the individuality 


SURVEY OF CELL-LIFE. 18] 


of the original cell is retained (independent cells), or as it is 
more or less completely lost (coalescing cells, and cells which 
undergo division). 

The varieties which occur amongst the independent cells, are 
partly of a chemical nature, and partly have reference to a dif- 
ference in the growth of the cell-membrane, by which means 
a change in the form of the cell may be produced. 

The cell-membrane differs in respect to its chemical quali- 
ties in different kinds of cells. That of the blood-corpuscles, 
for instance, is dissolved by acetic acid, whilst that of the 
cartilage-cell is not. The chemical composition of the cell- 
membrane differs even in the same cell according to its age, 
so that a transformation of the substance of the membrane 
itself takes place in plants; for, according to Schleiden, the 
cell-membrane of the youngest cells dissolves in water, the 
fully-developed cells not being acted upon by that fluid. The 
simple cells are still more remarkable for their cell-contents. 
One cell forms fat, another pigment, a third etherial oil; and 
here also a transformation of the cell-contents takes place. A 
granulous precipitate is seen to form gradually in what was in 
the first instance a pellucid cell, and this usually takes place first 
around the cell-nucleus ; or, vice versd, durimg incubation, the 
granulous (fatty) contents of the cells of the yelk-substance 
gradually undergo partial solution. According to Schleiden, 
this transformation of the substance of the cell-contents pro- 
ceeds in accordance with a certain rule; I have not made any 
investigations upon the subject in animals. 

We should also include under this head the formation of 
the secondary deposits upon the internal surface of the cell- 
membrane, so very frequently met with in plants. If a firm 
cohering substance be formed from the cell-contents, it may 
be deposited upon the internal surface of the cell-membrane. 
In plants this deposition generally takes place in layers, a 
stratum being formed in the first instance upon the internal 
surface of the cell-membrane, upon the internal surface of 
that one a second, and so on until at last the entire cavity 
may be almost filled by them. According to Valentin, these 
surrounding deposits always take place in spiral lines which 
are subject to great varieties in their arrangement, for there 
may be one or many of them, and they may either completely 


182 SURVEY OF CELL-LIFE. 


cover the internal surface of the cell-membrane, or not be in 
contact with each other at all. I have not observed any such 
secondary stratified depositions in animals. 

The variations which may occur in the growth of the cell- 
membrane in simple cells, depend upon the circumstance as to 
whether or not the addition of new molecules takes place 
equably at all parts of the cell-membrane. In the first case 
the form of the cell remains unchanged, and the only other 
distinction possible would be grounded upon the fact as to 
whether the greater part of the new molecules were deposited 
between the particles which lay side by side upon the super- 
ficies of the cell-membrane, or between those which lay one 
behind another in its thickness. The first mode of growth 
produces an expansion of the cell-membrane, the effect of the 
second is more especially to thicken it. Both modes are gene- 
rally combined, but in such a manner that the expansion of the 
cell-membrane prevails in most instances. 

A great variety of modifications in the form of the cells may 
be produced by the irregular distribution of the new mole- 
cules. The globular, which is their fundamental form, may 
be converted into a polyhedral figure, or the cells may become 
flattened into a round or oval or angular tablet, or the expan- 
sion of the cells may take place on one or on two opposite sides, 
so as to form a fibre, and these fibres again may either be flat, 
being at the same time in some instances serrated, or lastly, 
the expansion of the cells into fibres may take place on dif- 
ferent sides so as to give them the stellate form. Some of 
these changes of form are, no doubt, due to mechanical causes. 
Thus, for example, the polyhedral form is produced by the 
close crowding of the spherical cells, and these, when separated 
from one another, sometimes assume their round figure again ; 
such is the case with the yelk-cells. Some of the other 
changes would seem to be capable of explanation by exosmosis. 
If, for example, the contents of a round cell be so changed, 
that a fluid is generated in it which is less dense than the 
surrounding fluid, the cell will lose some of its contents by 
exosmosis, and must, therefore, collapse, and may become 
flattened into a table as in the blood-corpuscles. Such expla- 
nations, however, are unsatisfactory in by far the greatest num- 
ber of instances, and we are compelled to assume, that the 


SURVEY OF CELL-LIFE. 183 


growth does not necessarily proceed equably on all sides, but 
that the new molecules may be deposited in greater abundance 
in certain situations. Let us take the instance of a round 
_ cell, the cell-membrane of which is already developed, and 
suppose the deposition of new molecules to be confined to one 
particular part of the cell-membrane, that part would become 
expanded, and so a hollow fibre would grow forth from the cell, 
the cavity of which would communicate with the cell-cavity. 
The same result would take place, but more easily, if the new 
molecules were disposed unequally previous to the period when 
the external stratum of the precipitate, which is formed around 
the nucleus, had become condensed into a distinctly perceptible 
cell-membrane. The hollowing out of the fibre would then be 
less perfect, and the growth of the fibre must advance, particu- 
larly as regarded its thickness, before any manifest distinction 
between wall and cavity could be perceived. 

The cause of this irregular disposition of the new molecules 
may, in some instances, be due to circumstances altogether ex- 
ternal to the cell. If, for instance, a cell lay in such a position 
that one side of it was in contact with a concentrated nutri- 
tive material, one could conceive that side of the cell growing 
more vigorously, even though the force, which produces the 
growth of the cell, should operate equably throughout the entire 
cell. Such an explanation cannot, however, be received at all 
in most instances, but we must admit modifications in the 
principle of development of the cells, of such a nature, as that 
the force, which affects the general growth of the cells, is 
enabled to occasion an equable disposition of new molecules in 
one cell, and an unequal one in another. 

Amongst the changes which more or less completely deprive 
the original cells of their individuality, are to be classed, in the 
first place, the coalescence of the cell-walls with one another, 
or with the intercellular substance; secondly, the division of 
one cell into several; and, thirdly, the coalescence of several 
primary cells to form a secondary one. 

A coalescence of the cell-membrane with the intercellular sub- 
stance, or with a neighbouring cell-wall, appears to take place 
in some cartilages for example. At first the cell-membrane has | 
a sharply-defined external contour, by degrees the boundary 
line becomes paler, and at last is no longer perceptible with 


184 SURVEY OF CELL-LIFE. 


the microscope. We cannot, at present, lay down any general 
law respecting the circumstances under which such a coalescence 
occurs; it presupposes that the cell-membrane and intercellular 
substance are homogeneous structures, and may perhaps always 
take place when such a state exists. 

As regards the subdivision of the cells, we have already seen 
how a jutting out of the cell-membrane may be produced by 
its more vigorous growth in certain situations. But a jutting 
inwards into the cavity of the cell may also result from the 
very same process. Now, if we imagine this jutting inwards to 
take place in a circular form around a cell, as the consequence 
of a partial increase in the force of its growth, it may proceed 
to such an extent, that one cell may be separated into two, 
connected together only by a short peduncle, which may after- 
wards be absorbed. This would illustrate the most simple form 
of subdivision in a cell. In the animal cells, however, which 
undergo subdivision, that is, the fibre-cells, the process is more 
complicated; firstly, because when an elongated cell subdivides, 
it splits into many fibres; and, secondly, because the cells are so 
very minute. The process, therefore, cannot for these reasons 
be accurately traced, and the following is all that we can de- 
tect : a cell becomes elongated on two opposite sides into several 
fibres ; from the angle, which the fibres on either side form with 
each other, a striated appearance gradually extends over the 
body of the cell ; this formation of strize becomes more and more 
distinct, until the body of the cell splits entirely into fibres. 

The coalescence of several primary cells to form a secondary 
cell is, to a certain extent, the opposite process to the last. 
Several primary cells, of muscle for instance, are arranged close 
together in rows, and coalesce into a cylinder, in the thickness 
of which lie the nuclei of the primary cells. This cylinder is 
hollow and not interrupted by septa, and the nuclei lie upon the 
internal surface of its wall. These are the facts of the process, 
so far as they have as yet been observed. One can form a 
conception of so much as is yet required to render them com- 
plete. If two perfectly-developed cells coalesce together, their 
walls must first unite at the poimt of contact, and then the 
partition-wall between the cavities must be absorbed. Nature, 
however, does not by any means require that these acts should 
occur at precisely defined periods. The coalescence may take 


i 


SURVEY OF CELL-LIFE. 185 


place before the cell-wall and cell-cavity exist as distinct struc- 
tures, somewhat in the following manner: the nuclei are formed 
first, around them a new stratum of substance is deposited, the 
external portion of which, in accordance with the course of 
formation of an ordinary simple cell, would become condensed 
into a cell-membrane. But in this mstance the nuclei lie so 
close together, that the strata forming around them and corre- 
sponding to the cells, flow together, to form a cylinder, the ex- 
ternal portion of which becomes condensed into a membrane, 
just in the same manner as in simple cells, where merely the 
external portion of the stratum formed around the nucleus, 
becomes hardened on the outside into a membrane, in conse- 
quence of the reception of new molecules. There is, therefore, 
nothing in this which differs so very materially from the course 
of development of a simple cell; indeed, we seemed to be com- 
pelled to assume a similar process for the formation of the nuclei 
furnished with two or more nucleoli. (See page 176.) It is 
possible that there may be stages of transition between the 
ordinary simple cell and these secondary cells. It has been 
already mentioned at pages 117-118, that fat-cells occur in the 
cranial cavity of fishes, many of which contain two nuclei. 
It is possible that only one of them is the cytoblast of the 
cell, and that the second is a nucleus which has formed subse- 
quently ; but they resemble one another so completely in their 
characteristic position on the cell-membrane (see pl. III, fig. 
10,) that perhaps they may both be cytoblasts of a cell which 
has been formed around both nuclei, in consequence of the ex- 
ternal stratum of the precipitate having become condensed in 
such a manner that the membrane enclosed both nuclei. Mean- 
while observation affords no demonstrative proof on the sub- 
ject, and the similarity in the position of these two nuclei may 
be explained in another way. Fat thrusts all bodies which have 
imbibed water towards the outside of the cell, in order that it 
may assume its own globular form. If now a second nucleus 
should form in one of these fat-cells, it will be thrust towards 
the outside, and must gradually raise the cell-membrane into a 
prominence. It may also be observed, that opportunities of 
demonstrating the actual absorption of the fully-developed 
partition-wall between two cells do occur in the spiral vessels of 
plants. 


186 THEORY OF THE CELLS. 


THEORY OF THE CELLS. 


The whole of the foregoing investigation has been con- 
ducted with the object of exhibiting from observation alone the 
mode in which the elementary parts of organized bodies are 
formed. Theoretical views have been either entirely excluded, 
or where they were required (as in the foregoing retrospect of 
the cell-life), for the purpose of rendering facts more clear, or 
preventing subsequent repetitions, they have been so presented 
that it can be easily seen how much is observation and how 
much argument. But a question inevitably arises as to the 
basis of all these phenomena; and an attempt to solve it will 
be more readily permitted us, sce by making a marked sepa- 
ration between theory and observation the hypothetical may be 
clearly distinguished from that which is positive. An hypo- 
thesis is never prejudicial so long as we are conscious of the 
degree of reliance which may be placed upon it, and of the 
grounds on which it rests. Indeed it is advantageous, if not 
necessary for science, that when a certain series of phenomena 
is proved by observation, some provisional explanation should be 
conceived that will suit them as nearly as possible, even though 
it be in danger of being overthrown by subsequent observations; 
for it is only in this manner that we are rationally led to new 
discoveries, which either establish or refute the explanation. It 
is from this point of view I would beg that the following theory 
of organization may be regarded ; for the inquiry into the source 
of development of the elementary parts of organisms is, in fact, 
identical with the theory of organized bodies. 

The various opinions entertained with respect to the funda- 
mental powers of an organized body may be reduced to two, 
which are essentially different from one another. The first is, 
that every organism originates with an inherent power, which 
models it into conformity with a predominant idea, arranging 
the molecules in the relation necessary for accomplishing certain 
purposes held forth by this idea. Here, therefore, that which 
arranges and combines the molecules is a power acting with a 
definite purpose. A power of this kind would be essentially 
different from all the powers of inorganic nature, because action 


THEORY OF THE CELLS. 187 


goes on in the latter quite blindly. <A certain impression is 
followed of necessity by a certain change of quality and quantity, 
without regard to any purpose. In this view, however, the 
fundamental power of the organism (or the soul, in the sense 
employed by Stahl) would, inasmuch as it works with a definite 
individual purpose, be much more nearly allied to the im- 
material principle, endued with consciousness which we must 
admit operates in man. 

The other view is, that the fundamental powers of organized 
bodies agree essentially with those of imorganic nature, that 
they work altogether blindly according to laws of necessity and 
irrespective of any purpose, that they are powers which are as 
much established with the existence of matter as the phy- 
sical powers are. It might be assumed that the powers which 
form organized bodies do not appear at all in inorganic nature, 
because this or that particular combination of molecules, by 
which the powers are elicited, does not occur in inorganic 
nature, and yet they might not be essentially distinct from 
physical and chemical powers. It cannot, indeed, be denied 
that adaptation to a particular purpose, in some individuals 
even in a high degree, is characteristic of every organism ; 
but, according to this view, the source of this adaptation does 
not depend upon each organism being developed by the opera- 
tion of its own power in obedience to that purpose, but it 
originates as in organic nature, in the creation of the matter 
with its blind powers by a rational Being. We know, for 
instance, the powers which operate in our planetary system. 
They operate, like all physical powers, in accordance with 
blind laws of necessity, and yet is the planetary system re- 
markable for its adaptation to a purpose. The ground of 
this adaptation does not le in the powers, but in Him, who has 
so constituted matter with its powers, that in blindly obeying its 
laws it produces a whole suited to fulfil an imtended purpose. 
We may even assume that the planetary system has an indivi- 
dual adaptation to a purpose. Some external influence, such as 
a comet, may occasion disturbances of motion, without thereby 
bringing the whole into collision; derangements may occur on 
single planets, such as a high tide, &c., which are yet balanced 
entirely by physical laws. As respects their adaptation to a 
purpose, organized bodies differ from these in degree only; 


188 THEORY OF THE CELLS. 


and by this second view we are just as little compelled to 
conclude that the fundamental powers of organization operate 
according to laws of adaptation to a purpose, as we are in 
inorganic nature. 

The first view of the fundamental powers of organized bodies 
may be called the teleological, the second the physical view. 
An example will show at once, how important for physiology 
is the solution of the question as to which is to be followed. 
If, for instance, we define inflammation and suppuration to be 
the effort of the organism to remove a foreign body that has 
been introduced into it; or fever to be the effort of the or- 
ganism to eliminate diseased matter, and both as the result of 
the “autocracy of the organism,” then these explanations 
accord with the teleological view. For, since by these pro- 
cesses the obnoxious matter is actually removed, the process 
which effects them is one adapted to an end; and as the 
fundamental power of the organism operates in accordance with 
definite purposes, it may either set these processes in action 
primarily, or may also summon further powers of matter to its 
aid, always, however, remaining itself the ‘“ primum movens.” 
On the other hand, according to the physical view, this is just 
as little an explanation as it would be to say, that the motion 
of the earth around the sun is an effort of the fundamental 
power of the planetary system to produce a change of seasons 
on the planets, or to say, that ebb and flood are the reaction 
of the organism of the earth upon the moon. 

In physics, all those explanations which were suggested by 
a teleological view of nature, as “horror vacui,” and the like, 
have long been discarded. But in animated nature, adaptation 
—individual adaptation—to a purpose is so prominently 
marked, that it is difficult to reject all teleological explanations. 
Meanwhile it must be remembered that those explanations, 
which explain at once all and nothing, can be but the last 
resources, when no other view can possibly be adopted; and there 
is no such necessity for admitting the teleological view in the 
case of organized bodies. The adaptation to a purpose which 
is characteristic of organized bodies differs only in degree from 
what is apparent also in the inorganic part of nature; and the 
explanation that organized bodies are developed, like all the 
phenomena of inorganic nature, by the operation of blind laws 


THEORY OF THE CELLS. 189 


framed with the matter, cannot be rejected as impossible. 
Reason certainly requires some ground for such adaptation, but 
for her it is sufficient to assume that matter with the powers 
inherent in it owes its existence to a rational Being. Once esta- 
blished and preserved in their integrity, these powers may, in 
accordance with their immutable laws of blind necessity, very 
well produce combinations, which manifest, even in a high degree, 
individual adaptation to a purpose. If, however, rational power 
interpose after creation merely to sustain, and not as an imme- 
ciately active agent, it may, so far as natural science is concerned, 
be entirely excluded from the consideration of the creation. 

But the teleological view leads to further difficulties in the 
explanation, and especially with respect to generation. If we 
assume each organism to be formed by a power which acts 
according to a certain predominant idea, a portion of this power 
may certainly reside in the ovum during generation ; but then 
we must ascribe to this subdivision of the original power, at 
the separation of the ovum from the body of the mother, the 
capability of producing an organism similar to that which the 
power, of which it is but a portion, produced: that is, we must 
assume that this power is infinitely divisible, and yet that each 
part may perform the same actions as the whole power. If, 
on the other hand, the power of organized bodies reside, like 
the physical powers, in matter as such, and be set free only 
by a certain combination of the molecules, as, for instance, 
electricity is set free by the combination of a zine and copper 
plate, then also by the conjunction of molecules to form an 
ovum the power may be set free, by which the ovum is capable 
of appropriating to itself fresh molecules, and these newly- 
conjoined molecules again by this very mode of combination 
acquire the same power to assimilate fresh molecules. The 
first development of the many forms of organized bodies—the 
progressive formation of organic nature indicated by geology— 
is also much more difficult to understand according to the 
teleological than the physical view. 

Another objection to the teleological view may be drawn 
from the foregoing investigation. The molecules, as we have 
seen, are not immediately combined in various ways, as the 
purpose of the organism requires, but the formation of the 
elementary parts of organic bodies is regulated by laws which 


190 THEORY OF THE CELLS. 


are essentially the same for all elementary parts. One can 
see no reason why this should be the case, if each organism be 
endued with a special power to frame the parts according to 
the purpose which they have to fulfil: it might much rather 
be expected that the formative principle, although identical 
for organs physiologically the same, would yet in different 
tissues be correspondingly varied. This resemblance of the 
elementary parts has, in the imstance of plants, already led to 
the conjecture that the cells are really the organisms, and that 
the whole plant is an aggregate of these organisms arranged 
according to certain laws. But since the elementary parts of 
animals bear exactly similar relations, the mdividuality of an 
entire animal would thus be lost; and yet precisely upon the 
individuality of the whole animal does the assumption rest, that 
it possesses a single fundamental power operating in accordance 
with a definite idea. 

Meanwhile we cannot altogether lay aside teleological views 
if all phenomena are not clearly explicable by the physical view. 
It is, however, unnecessary to do so, because an explanation, 
according to the teleological view, is only admissible when the 
physical can be shown to be impossible. In any case it con- 
duces much more to the object of science to strive, at least, to 
adopt the physical explanation. And I would repeat that, 
when speaking of a physical explanation of organic phenomena, 
it is not necessary to understand an explanation by known 
physical powers, such, for instance, as that universal refuge 
electricity, and the like; but an explanation by means of 
powers which operate like the physical powers, in accordance 
with strict laws of blind necessity, whether they be also to be 
found in inorganic nature or not. 

We set out, therefore, with the supposition that an organized 
body is not produced by a fundamental power which is guided 
in its operation by a definite idea, but is developed, according 
to blind laws of necessity, by powers which, like those of 
inorganic nature, are established by the very existence of 
matter. As the elementary materials of organic nature are 
not different from those of the morganic kingdom, the source 
of the organic phenomena can only reside in another combi- 
nation of these materials, whether it be in a peculiar mode of 
union of the elementary atoms to form atoms of the second 


THEORY OF THE CELLS. 191 


order, or in the arrangement of these conglomerate molecules 
when forming either the separate morphological elementary 
parts of organisms, or an entire organism. We have here 
to do with the latter question solely, whether the cause of 
organic phenomena lies in the whole organism, or in its sepa- 
rate elementary parts. If this question can be answered, a 
further inquiry still remains as to whether the organism or its 
elementary parts possess this power through the peculiar mode 
of combination of the conglomerate molecules, or through the 
mode in which the elementary atoms are united into con- 
glomerate molecules. 

We may, then, form the two following ideas of the cause of 
organic phenomena, such as growth, &c. First, that the cause 
resides in the totality of the organism. By the combination 
of the molecules into a systematic whole, such as the organism 
is in every stage of its development, a power is engendered, 
which enables such an organism to take up fresh material from 
without, and appropriate it either to the formation of new 
elementary parts, or to the growth of those already present. 
Here, therefore, the cause of the growth of the elementary 
parts resides in the totality of the organism. The other mode 
of explanation is, that growth does not ensue from a power 
resident in the entire organism, but that each separate ele- 
mentary part is possessed of an independent power, an inde- 
pendent life, so to speak ; in other words, the molecules in each 
separate elementary part are so combined as to set free a 
power by which it is capable of attracting new molecules, and 
so increasing, and the whole organism subsists only by means 
of the reciprocal’ action of the single elementary parts. So 
that here the single elementary parts only exert an active 
influence on nutrition, and totality of the organism may indeed 
be a condition, but is not in this view a cause. 

In order to determine which of these two views is the cor- 
rect one, we must summon to our aid the results of the pre- 
vious investigation. We have seen that all organized bodies 
are composed of essentially similar parts, namely, of cells ; 
that these cells are formed and grow in accordance with essen- 


1 The word ‘reciprocal action” must here be taken in its widest sense, as 


implying the preparation of material by one elementary part, which another requires 
for its own nutrition. 


192 THEORY OF THE CELLS. 


tially similar laws; and, therefore, that these processes must, 
in every instance, be produced by the same powers. Now, if we 
find that some of these elementary parts, not differing from 
the others, are capable of separating themselves from the 
organism, and pursuing an independent growth, we may thence 
conclude that each of the other elementary parts, each cell, 
is already possessed of power to take up fresh molecules and 
grow; and that, therefore, every elementary part possesses a 
power of its own, an independent life, by means of which it 
would be enabled to develop itself independently, if the relations 
which it bore to external parts were but similar to those in 
which it stands in the organism. The ova of animals afford 
us examples of such independent cells, growing apart from the 
organism. It may, indeed, be said of the ova of higher animals, 
that after impregnation the ovum is essentially different from 
the other cells of the organism; that by impregnation there 
is a something conveyed to the ovum, which is more to it than 
an external condition for vitality, more than nutrient matter ; 
and that it might thereby have first received its peculiar 
vitality, and therefore that nothing can be inferred from it with 
respect to the other cells. But this fails in application to those 
classes which consist only of female individuals, as well as with 
the spores of the lower plants ; and, besides, in the inferior 
plants any given cell may be separated from the plant, and 
then grow alone. So that here are whole plants consisting 
of cells, which can be positively proved to have independent 
vitality. Now, as all cells grow according to the same laws, 
and consequently the cause of growth cannot in one case lie 
in the cell, and in another in the whole organism ; and since 
it may be further proved that some cells, which do not differ 
from the rest in their mode of growth, are developed indepen- 
dently, we must ascribe to all cells an independent vitality, that 
is, such combinations of molecules as occur in any single cell, 
are capable of setting free the power by which it is enabled 
to take up fresh molecules. The cause of nutrition and 
growth resides not in the organism as a whole, but in the 
separate elementary parts—the cells. The failure of growth 
in the case of any particular cell, when separated from an 
organized body, is as slight an objection to this theory, as it 
is an objection against the independent vitality of a bee, that 


THEORY OF THE CELLS. 193 


it cannot continue long in existence after being separated 
from its swarm. The manifestation of the power which resides 
in the cell depends upon conditions to which it is subject only 
when in connexion with the whole (organism). 

The question, then, as to the fundamental power of orga- 
nized bodies resolves itself into that of the fundamental powers 
of the individual cells. We must now consider the general 
phenomena attending the formation of cells, in order to dis- 
cover what powers may be presumed to exist in the cells to 
explain them. ‘These phenomena may be arranged in two 
natural groups: first, those which relate to the combination of 
the molecules to form a cell, and which may be denominated the 
plastic phenomena of the cells; secondly, those which result 
from chemical changes either in the component particles of the 
cell itself, or in the surrounding cytoblastema, and which may 
be called metabolic phenomena (70 perafoAtkov, implying that 
which is liable to occasion or to suffer change). 

The general plastic appearances in the cells are, as we have 
seen, the following: at first a minute corpuscle is formed, 
(the nucleolus) ; a layer of substance (the nucleus) is then pre- 
cipitated around it, which becomes more thickened and ex- 
panded by the continual deposition of fresh molecules between 
those already present. Deposition goes on more vigorously at 
the outer part of this layer than at the inner. Frequently the 
entire layer, or in other imstances the outer part of it only, 
becomes condensed to a membrane, which may continue to take 
up new molecules in such a manner that it increases more 
rapidly in superficial extent than in thickness, and thus an 
intervening cavity is necessarily formed between it and the 
nucleolus. A second layer (cell) is next precipitated around 
this first, im which precisely the same phenomena are repeated, 
with merely the difference that in this case the processes, espe- 
cially the growth of the layer, and the formation of the space 
intervening between it and the first layer (the cell-cavity), go 
on more rapidly and more completely. Such were the pheno- 
mena in the formation of most cells; m some, however, there 
appeared to be only a single layer formed, while in others (those 
especially in which the nucleolus was hollow) there were three. 
The other varieties in the development of the elementary parts 
were (as we saw) reduced to these—that if two neighbouring 

13 


194 THEORY OF THE CELLS. 


cells commence their formation so near to one another that the 
boundaries of the layers forming around each of them meet at 
any spot, a common layer may be formed enclosing the two 
incipient cells. So at least the origin of nuclei, with two or 
more nucleoli, seemed explicable, by a coalescence of the first 
layers (corresponding to the nucleus), and the union of many 
primary cells imto one secondary cell by a similar coalescence 
of the second layers (which correspond to the cell). But the 
further development of these common layers proceeds as though 
they were only an ordinary single layer. Lastly, there were 
some varieties in the progressive development of the cells, which 
were referable to an unequal deposition of the new molecules 
between those already present in the separate layers. In this 
way modifications of form and division of the cells were ex- 
plained. And among the number of the plastic phenomena in 
the cells we may mention, lastly, the formation of secondary 
deposits ; for stances occur in which one or more new layers, 
each on the inner surface of the previous one, are deposited on 
the inner surface of a simple or of a secondary cell. 

These are the most important phenomena observed in the 
formation and development of cells. The unknown cause, 
presumed to be capable of explaining these processes in the 
cells, may be called the plastic power of the cells. We will, in 
the next place, proceed to determine how far a more accurate 
definition of this power may be deduced from these phenomena. 

In the first place, there is a power of attraction exerted in 
the very commencement of the cell, in the nucleolus, which 
occasions the addition of new molecules to those already pre- 
sent. We may imagine the nucleolus itself to be first formed 
by a sort of crystallization from out of a concentrated fluid. 
For if a fluid be so concentrated that the molecules of the 
substance in solution exert a more powerful mutual attraction 
than is exerted between them and the molecules of the fluid 
in which they are dissolved, a part of the solid substance must 
be precipitated. One can readily understand that the fluid 
must be more concentrated when new cells are being formed in 
it than when those already present have merely to grow. For 
if the cell is already partly formed, it exerts an attractive force 
upon the substance still in solution. There is then a cause 
for the deposition of this substance, which does not co-operate 


. 
; 
: 


THEORY OF THE CELLS. 195 


when no part of the cell is yet formed. Therefore, the greater 
the attractive force of the cell is, the less concentration of the 
fluid is required; while, at the commencement of the forma- 
tion of a cell, the fluid must be more than concentrated. But 
the conclusion which may be thus directly drawn, as to the 
attractive power of the cell, may also be verified by observation. 
Wherever the nutrient fluid is not equally distributed in a 
tissue, the new cells are formed in that part into which the 
fluid penetrates first, and where, consequently, it is most con- 
centrated. Upon this fact, as we have seen, depended the 
difference between the growth of organized and unorganized 
tissues (see page 169). And this confirmation of the foregoing 
conclusion by experience speaks also for the correctness of the 
reasoning itself. 

The attractive power of the cells operates so as to effect the 
addition of new molecules in two ways,—first, in layers, and 
secondly, in such a manner in each layer that the new mole- 
cules are deposited between those already present. This is 
only an expression of the fact ; the more simple law, by which 
several layers are formed and the molecules are not all de- 
posited between those already present, cannot yet be explained. 
The formation of layers may be repeated once, twice, or thrice. 
The growth of the separate layers is regulated by a law, that 
the deposition of new molecules should be greatest at the part 
where the nutrient fluid is most concentrated. Hence the 
outer part particularly becomes condensed into a membrane 
both in the layer corresponding to the nucleus and in that 
answering to the cell, because the nutrient fluid penetrates 
from without, and consequently is more concentrated at the 
outer than at the imner part of each layer. For the same 
reason the nucleus grows rapidly, so long as the layer of the 
cell is not formed around it, but it either stops growing 
altogether, or at least grows much more slowly so soon as 
the cell-layer has surrounded it; because then the latter 
receives the nutrient matter first, and, therefore, in a more 
concentrated form. And hence the cell becomes, in a general 
sense, much more completely developed, while the nucleus- 
layer usually remains at a stage of development, in which 
the cell-layer had been in its earlier period. The addition of 
new molecules is so arranged that the layers imcrease more 


196 THEORY OF THE CELLS. 


considerably in superficial extent than in thickness; and thus 
an intervening space is formed between each layer and the 
one preceding it, by which cells and nuclei are formed into 
actual hollow vesicles. From this it may be inferred that 
the deposition of new molecules is more active between those 
which lie side by side along the surface of the membrane, than 
between those which lie one upon the other in its thickness, 
Were it otherwise, each layer would increase in thickness, but 
there would be no intervening cavity between it and the pre- 
vious one, there would be no vesicles, but a solid body com- 
posed of layers. 

Attractive power is exerted in all the solid parts of the cell. 
This follows, not only from the fact that new molecules may 
be deposited everywhere between those already present, but 
also from the formation of secondary deposits. When the 
cavity of a cell is once formed, material may be also attracted 
from its contents and deposited in layers; and as this depo- 
sition takes place upon the imner surface of the membrane 
of the cell, it is probably that which exerts the attractive in- 
fluence. This formation of layers on the inner surface of the 
cell-membrane is, perhaps, merely a repetition of the same 
process by which, at an earlier period, nucleus and cell were 
precipitated as layers around the nucleolus. It must, how- 
ever, be remarked that the identity of these two processes 
cannot be so clearly proved as that of the processes by which 
nucleus and cell are formed; more especially as there is a 
variety in the phenomena, for the secondary deposits in plants 
occur in spiral forms, while this has at least not yet been de- 
monstrated in the formation of the cell-membrane and the 
nucleus, although by some botanical writers the cell-membrane 
itself is supposed to consist of spirals. 

The power of attraction may be uniform throughout the 
whole cell, but it may also be confined to single spots; the 
deposition of new molecules is then more vigorous at these 
spots, and the consequence of this uneven growth of the cell- 
membrane is a change in the form of the cell. 

The attractive power of the cells manifests a certain form of 
election in its operation. It does not take up all the substances 
contained in the surrounding cytoblastema, but only particular 
ones, either those which are analogous with the substance 


THEORY OF THE CELLS. 197 


already present in the cell (assimilation), or such as differ from 
it in chemical properties. The several layers grow by assimi- 
lation, but when a new layer is being formed, different material 
from that of the previously-formed layer is attracted: for the 
nucleolus, the nucleus and cell-membrane are composed of 
materials which differ in their chemical properties. 

Such are the peculiarities of the plastic power of the cells, 
so far as they can as yet be drawn from observation. But 
the manifestations of this power presuppose another faculty of 
the cells. The cytoblastema, in which the cells are formed, 
contains the elements of the materials of which the cell is 
composed, but in other combinations: it is not a mere solu- 
tion of cell-material, but it contains only certain organic 
substances in solution. The cells, therefore, not only attract 
materials from out of the cytoblastema, but they must have 
the faculty of producing chemical changes in its constituent 
particles. Besides which, all the parts of the cell itself may be 
chemically altered during the process of its vegetation. The 
unknown cause of all these phenomena, which we comprise 
under the term metabolic phenomena of the cells, we will 
denominate the metabolic power. 

The next point which can be proved is, that this power is 
an attribute of the cells themselves, and that the cytoblastema 
is passive under it. We may mention vinous fermentation’ 


' I could not avoid bringing forward fermentation as an example, because it is 
the best known illustration of the operation of the cells, and the simplest represen- 
tation of the process which is repeated in each cell of the living body. Those 
who do not as yet admit the theory of fermentation set forth by Cagniard-Latour, 
and myself, may take the development of any simple cells, especially of the spores, 
as an example; and we will in the text draw no conclusion from fermentation 
which cannot be proved from the development of other simple cells which grow 
independently, particularly the spores of the inferior plants. We have every con- 
ceivable proof that the fermentation-granules are fungi. Their form is that of fungi; 
in structure they, like them, consist of cells, many of which enclose other young cells. 
They grow, like fungi, by the shooting forth of new cells at their extremities; they 
propagate like them, partly by the separation of distinct cells, and partly by the gene- 
ration of new cells within those already present, and the bursting of the parent-cells. 
Now, that these fungi are the cause of fermentation, follows, first, from the constancy 
of their occurrence during the process; secondly, from the cessation of fermentation 
under any influences by which they are known to be destroyed, especially boiling heat, 
arseniate of potass, &c. ; and, thirdly, because the principle which excites the process 
of fermentation must be a substance which is again generated and increased by the 


198 THEORY OF THE CELLS. 


as an instance of this. A decoction of malt will remain for 
a long time unchanged; but as soon as some yeast is added 
to it, which consists partly of entire fungi and partly of a 
number of single cells, the chemical change immediately ensues. 
Here the decoction of malt is the cytoblastema; the cells clearly 
exhibit activity, the cytoblastema, in this instance even a boiled 
fluid, being quite passive during the change. The same occurs 
when any simple cells, as the spores of the lower plants, are 
sown in boiled substances. 

In the cells themselves again, it appears to be the solid 
parts, the cell-membrane and the nucleus, which produce the 
change. The contents of the cell undergo similar and even 
more various changes than the external cytoblastema, and it is 
at least probable that these changes originate with the solid 
parts composing the cells, especially the cell-membrane, because 
the secondary deposits are formed on the inner surface of the 
cell-membrane, and other precipitates are generally formed in 
the first instance around the nucleus. It may therefore, on the 
whole, be said that the solid component particles of the cells 
possess the power of chemically altering the substances in con- 
tact with them. 

The substances which result from the transformation of the 


process itself, a phenomenon which is met with only in living organisms. Neither do 
{ see how any further proof can possibly be obtained otherwise than by chemical ana- 
lysis, unless it can be proved that the carbonic acid and alcohol are formed only at 
the surface of the fungi. I have made a number of attempts to prove this, but they 
have not as yet completely answered the purpose. A long test-tube was filled with 
a weak solution of sugar, coloured of a delicate blue with litmus, and a very small 
quantity of yeast was added to it, so that fermentation might not begin until several 
hours afterwards, and the fungi, haying thus previously settled at the bottom, the fluid 
might become clear. When the carbonic acid (which remained in solution) commenced 
to be formed, the reddening of the blue fluid actually began at the bottom of the tube. 
If at the beginning a rod were put into the tube, so that the fungi might settle upon 
it also, the reddening began both at the bottom, and upon the rod. This proves, 
at least, that an undissolved substance which is heavier than water gives rise to 
fermentation ; and the experiment was next repeated on a small scale under the 
microscope, to see whether the reddening really proceeded from the fungi, but the 
colour was too pale to be distinguished, and when the fluid was coloured more 
deeply no fermentation ensued; meanwhile, it is probable that a reagent upon car- 
bonic acid may be found which will serve for microscopic observation, and not 
interrupt fermentation. The foregoing inquiry into the process by which organized 
bodies are formed, may perhaps, however, serve in some measure to recommend this 
theory of fermentation to the attention of chemists. 


THEORY OF THE CELLS. 199 


contents of the cell are different from those which are produced 
by change in the external cytoblastema. What is the cause 
of this difference, if the metamorphosing power of the cell- 
membrane be limited to its immediate neighbourhood merely ? 
Might we not much rather expect that converted substances 
would be found without distinction on the inner as on the 
outer surface of the cell-membrane? It might be said that the 
cell-membrane converts the substance in contact with it without 
distinction, and that the variety in the products of this con- 
version depends only upon a difference between the convertible 
substance contained in the cell and the external cytoblastema. 
But the question then arises, as to how it happens that the 
contents of the cell differ from the external cytoblastema. If 
it be true that the cell-membrane, which at first closely sur- 
rounds the nucleus, expands in the course of its growth, so as 
to leave an interspace between it and the cell, and that the 
contents of the cell consist of fluid which has entered this 
space merely by imbibition, they cannot differ essentially from 
the external cytoblastema. I think therefore that, in order to 
explain the distinction between the cell-contents and the ex- 
ternal cytoblastema, we must ascribe to the cell-membrane not 
only the power in general of chemically altering the substances 
which it is either in contact with, or has imbibed, but also of 
so separating them that certain substances appear on its inner, 
and others on its outer surface. The secretion of substances 
already present in the blood, as, for instance, of urea, by the 
cells with which the urinary tubes are lined, cannot be ex- 
plained without such a faculty of the cells. There is, however, 
nothing so very hazardous in it, since it is a fact that different 
substances are separated in the decompositions produced by 
the galvanic pile. It might perhaps be conjectured from this 
peculiarity of the metabolic phenomena in the cells, that a 
particular position of the axes of the atoms composing the cell- 
membrane is essential for the production of these appearances. 

Chemical changes occur, however, not only in the cytobla- 
stema and the cell-contents, but also in the solid parts of 
which the cells are composed, particularly the cell-membrane. 
Without wishing to assert that there is any intimate connexion 
between the metabolic power of the cells and galvanism, I may 
yet, for the sake of making the representation of the process 


200 THEORY OF THE CELLS. 


more clear, remark that the chemical changes produced by a 
galvanic pile are accompanied by corresponding changes in the 
pile itself. 

The more obscure the cause of the metabolic phenomena in 
the cells is, the more accurately we must mark the circum- 
stances and phenomena under which they occur, One condi- 
tion to them is a certain temperature, which has a maximum 
and a minimum. The phenomena are not produced in a 
temperature below 0° or above 80° R.; boiling heat destroys 
this faculty of the cells permanently ; but the most favorable 
temperature is one between 10° and 32° R. Heat is evolved 
by the process itself. 

Oxygen, or carbonic acid, in a gaseous form or lightly con- 
fined, is essentially necessary to the metabolic phenomena of 
the cells. The oxygen disappears and carbonic acid is formed, 
or vice versa, carbonic acid disappears, and oxygen is formed. 
The universality of respiration is based entirely upon this 
fundamental condition to the metabolic phenomena of the 
cells. It is so important that, as we shall see further on, 
even the principal varieties of form in organized bodies are 
occasioned by this peculiarity of the metabolic process in the 
cells. 

Each cell is not capable of producing chemical changes in 
every organic substance contained in solution, but only in par- 
ticular ones. The fungi of fermentation, for mstance, effect 
no changes in any other solutions than sugar; and the spores 
of certain plants do not become developed in all substances. 
In the same manner it is probable that each cell in the animal 
body converts only particular constituents of the blood. 

The metabolic power of the cells is arrested not only by 
powerful chemical actions, such as destroy organic substances 
in general, but also by matters which chemically are less un- 
congenial ; for instance, concentrated solutions of neutral salts. 
Other substances, as arsenic, do so in less quantity. The meta- 
bolic phenomena may be altered in quality by other substances, 
both organic and inorganic, and a change of this kind may re- 
sult even from mechanical impressions on the cells. 

Such are the most essential characteristics of the funda- 
mental powers of the cells, so far as they can as yet be deduced 
from the phenomena. And now, in order to comprehend dis- 


THEORY OF THE CELLS. 201 


tinctly in what the peculiarity of the formative process of a 
cell, and therefore in what the peculiarity of the essential 
phenomenon in the formation of organized bodies cousists, we 
will compare this process with a phenomenon of inorganic 
nature as nearly as possible similar to it. Disregarding all 
that is specially peculiar to the formation of cells, in order to 
find a more general definition in which it may be included with 
a process occurring in inorganic nature, we may view it as a 
process in which a solid body of definite and regular shape is 
formed in a fluid at the expense of a substance held in solu- 
tion by that fluid. The process of crystallization in inorganic 
nature comes also within this definition, and is, therefore, the 
nearest analogue to the formation of cells. 

Let us now compare the two processes, that the difference 
of the organic process may be clearly manifest. First, with 
reference to the plastic phenomena, the forms of cells and 
crystals are very different. The primary forms of crystals 
are sumple, always angular, and bounded by plane surfaces ; 
they are regular, or at least symmetrical, and even the very 
varied secondary forms of crystals are almost, without exception, 
bounded by plane surfaces. But manifold as is the form of 
cells, they have very little resemblance to crystals; round 
surfaces predominate, and where angles occur, they are never 
quite sharp, and the polyhedral crystal-like form of many cells 
results only from mechanical causes. The structure too of cells 
and of crystals is different. Crystals are solid bodies, composed 
merely of layers placed one upon another; cells are hollow 
vesicles, either single, or several inclosed one within another. 
And if we regard the membranes of these vesicles as layers, 
there will still remain marks of difference between them and 
crystals ; these layers are not in contact, but contain fluid be- 
tween them, which is not the case with crystals; the layers in 
the cells are few, from one to three only; and they differ from . 
each other in chemical properties, while those of crystals con- 
sist of the same chemical substance. Lastly, there is also a 
great difference between crystals and cells in their mode of 
growth. Crystals grow by apposition, the new molecules are 
set only upon the surface of those already deposited, but cells 
increase also by intussusception, that is to say, the new mole- 
cules are deposited also between those already present. 


202 THEORY OF THE CELLS. 


But greatly as these plastic phenomena differ in cells and 
in crystals, the metabolic are yet more different, or rather they 
are quite peculiar to cells. For a crystal to grow, it must be 
already present as such in the solution, and some extraneous 
cause must interpose to diminish its solubility. Cells, on the 
contrary, are capable of producing a chemical change in the 
surrounding fluid, of generating matters which had not pre- 
viously existed in it as such, but of which only the elements 
were present in another combination. They therefore require 
no extraneous influence to effect a change of solubility ; for if 
they can produce chemical changes in the surrounding fluid, 
they may also produce such substances as could not be held in 
solution under the existing circumstances, and therefore need 
no external cause of growth. If a crystal be laid in a pretty 
strong solution, of a substance similar even to itself, nothing 
ensues without our interference, or the crystal dissolves com- 
pletely: the fluid must be evaporated for the crystal to in- 
crease. If a cell be laid in a solution of a substance, even 
different from itself, it grows and converts this substance 
without our aid. And this it is from which the process going 
on in the cells (so long as we do not separate it into its several 
acts) obtains that magical character, to which attaches the idea 
of Life. 

From this we perceive how very different are the phenomena 
in the formation of cells and of crystals. Meanwhile, however, 
the points of resemblance between them should not he over- 
looked. They agree in this important point, that solid bodies 
of a certain regular shape are formed in obedience to definite 
laws at the expense of a substance contained im solution in a 
fluid; and the crystal, like the cell, is so far an active and posi- 
tive agent as to cause the substances which are precipitated to 
be deposited on itself, and nowhere else. We must, therefore, 
_attribute to it as well as to the cell a power to attract the sub- 
stance held in solution in the surrounding fluid. It does not 
indeed follow that these two attractive powers, the power of 
crystallization—to give it a brief title—and the plastic power 
of the cells are essentially the same. ‘This could only be ad- 
mitted, if it were proved that both powers acted according to 
the same laws. But this is seen at the first glance to be by 
no means the case: the phenomena in the formation of cells 


| 
. 
. 


THEORY OF THE CELLS. 203 


and crystals, are, as we have observed, very different, even if 
we regard merely the plastic phenomena of the cells, and leave 
their metabolic power (which may possibly arise from some 
other peculiarity of organic substance) for a time entirely out 
of the question. 

Is it, however, possible that these distinctions are only 
secondary, that the power of crystallization and the plastic 
power of the cells are identical, and that an original difference 
can be demonstrated between the substance of cells and that 
of crystals, by which we may perceive that the substance of 
cells must crystallize as cells according to the laws by which 
crystals are formed, rather than in the shape of the ordinary 
crystals? It may be worth while to institute such an inquiry. 

In seeking such a distinction between the substance of cells 
and that of crystals, we may say at once that it cannot con- 
sist in anything which the substance of cells has in common 
with those organic substances which crystallize in the ordinary 
form. Accordingly, the more complicated arrangement of the 
atoms of the second order in organic bodies cannot give rise to 
this difference ; for we see in sugar, for instance, that the mode 
of crystallization is not altered by this chemical composition. 

Another point of difference by which inorganic bodies are 
distinguished from at least some of the organic bodies, is 
the faculty of imbibition. Most organic bodies are capable 
of being infiltrated by water, and in such a manner that it 
penetrates not so much into the interspaces between the ele- 
mentary tissues of the body, as into the simple structureless 
tissues, such as areolar tissue, &c.; so that they form an 
homogeneous mixture, and we can neither distinguish par- 
ticles of organic matter, nor interspaces filled with water. The 
water occupies the infiltrated organic substances, just as it is 
present in a solution, and there is as much difference between 
the capacity for imbibition and capillary permeation, as 
there is between a solution and the phenomena of capillary 
permeation. When water soaks through a layer of glue, we 
do not imagine it to pass through pores, in the common sense 
of the term; and this is just the condition of all substances 
capable of imbibition. They possess, therefore, a double 
nature, they have a definite form like solid bodies; but like 
fluids, on the other hand, they are also permeable by anything 


204 THEORY OF THE CELLS. 


held in solution. As a specifically lighter fluid poured on one 
specifically heavier so carefully as not to mix with it, yet gra- 
dually penetrates it, so also, every solution, when brought into 
contact with a membrane already infiltrated with water, bears 
the same relations to the membrane, as though it were a solu- 
tion. And crystallization being the transition from the fluid to 
the solid state, we may conceive it possible, or even probable, 
that if bodies, capable of existing in an intermediate state 
between solid and fluid could be made to crystallize, a con- 
siderable difference would be exhibited from the ordinary mode 
of crystallization. In fact, there is nothing, which we call a 
crystal, composed of substance capable of imbibition ; and even 
among organized substances, crystallization takes place only in 
those which are capable of imbibition, as fat, sugar, tartaric 
acid, &e. The bodies capable of imbibition, therefore, either 
do not crystallize at all, or they do so under a form so different 
from the crystal, that they are not recognized as such. 

Let us inquire what would most probably ensue, if material 
capable of imbibition crystallized according to the ordinary 
laws, what varieties from the common crystals would be most 
likely to show themselves, assuming only that the solution has 
permeated through the parts of the crystal already formed, 
and that new molecules can therefore be deposited between 
them. The ordinary crystals increase only by apposition ; but 
there may be an important difference in the mode of this 
apposition. If the molecules were all deposited symmetrically 
one upon another, we might indeed have a body of a certain 
external form like a crystal; but it would not have the struc- 
ture of one, it would not consist of layers. The existence 
of this laminated structure in crystals presupposes a double 
kind of apposition of their molecules; for in each layer the 
newly-deposited molecules coalesce, and become continuous 
with those of the same layer already present ; but those mole- 
cules which form the adjacent surfaces of two layers do not 
coalesce. This is a remarkable peculiarity in the formation of 
crystals, and we are quite ignorant of its cause. We cannot 
yet perceive why the new molecules, which are being deposited 
on the surface of a crystal (already formed up to a certain 
point), do not coalesce and become continuous with those 
already deposited, like the molecules in each separate layer, 


THEORY OF THE CELLS. 205 


instead of forming, as they do, a new layer; and why this new 
layer does not constantly increase in thickness, instead of pro- 
ducimg a second layer around the crystal, and so on. In the 
meantime we can do no more than express the fact in the form 
of a law, that the coalescing molecules are deposited rather along 
the surface beside each other, than in the thickness upon one 
another, and thus, as the breadth of the layer depends upon the 
size of the crystal, so also the layer can attain only a certain 
thickness, and beyond this, the molecules which are being de- 
posited cannot coalesce with it, but must form a new layer. 

If we now assume that bodies capable of imbibition could 
also crystallize, the two modes of junction of the molecules 
should be shown also by them. Their structure should also 
be laminated, at least there is no perceptible reason for 
a difference in this particular, as the very fact of layers 
bemg formed in common crystals shows that the molecules 
need not be all joined together in the most exact manner 
possible. The closest possible conjunction of the molecules 
takes place only in the separate layers. In the common 
crystals this occurs by apposition of the new molecules on 
the surface of those present and eoalescence with them. In 
bodies capable of imbibition, a much closer union is possible, 
because in them the new molecules may be deposited by intus- 
susception between those already present. It is scarcely, 
therefore, too bold an hypothesis to assume, that when bodies 
capable of imbibition crystallize, their separate layers would 
increase by intussusception ; and that this does not happen in 
ordinary crystals, simply because it is impossible. 

Let us then imagine a portion of the crystal to be formed : 
new molecules continue to be deposited, but do not coalesce 
with the portion of the crystal already formed; they unite with 
one another only, and form a new layer, which, according to 
analogy with the common crystals, may invest either the whole 
or apart of the crystal. We will assume that it invests the 
entire crystal. Now, although this layer be formed by the 
deposition of new molecules between those already present in- 
stead of by apposition, yet this does not involve any change in 
the law, in obedience to which the deposition of the coalescing 
molecules goes on more vigorously in two directions, that is, 
along the surface, than it does in the third direction corre- 


206 THEORY OF THE CELLS. 


sponding to the thickness of the layer; that is to say, the 
molecules which are deposited by intussusception between those 
already present, must be deposited much more vigorously be- 
tween those lying together along the surface of the layer than 
between those which lie over one another in its thickness. 
This deposition of molecules side by side is limited in common 
crystals by the size of the crystal, or by that of the surface on 
which the layer is formed; the coalescence of molecules there- 
fore ceases as regards that layer, and a new one begins. But 
if the layers grow by intussusception in crystals capable of 
imbibition, there is nothing to prevent the deposition of more 
molecules between those which lie side by side upon the sur- 
face, even after the lamina has invested the whole crystal ; it 
may continue to grow without the law by which the new mole- 
cules coalesce requiring to be altered. But the consequence is, 
that the layer becomes, in the first instance more condensed, 
that is, more solid substance is taken into the same space ; 
and afterwards it will expand and separate from the completed 
part of the crystal so as to leave a hollow space between itself 
and the crystal; this space fills with fluid by imbibition, and 
the first-formed portion of the crystal adheres to a spot on its 
inner surface. Thus, in bodies capable of imbibition, mstead 
of a new layer attached to the part of the crystal already 
formed, we obtain a hollow vesicle. At first this must have the 
shape of the body of the crystal around which it is formed, 
and must, therefore, be angular, if the crystal is angular. If, 
however, we imagine this layer to be composed of soft sub- 
stance capable of imbibition, we may readily comprehend how 
such a vesicle must very soon become round or oval. But the 
first formed part of the crystal also consists of substance capable 
of imbibition, so that it is very doubtful whether it must have 
an angular form at all. In common crystals atoms of some 
one particular substance are deposited together, and we can 
understand how a certain angular form of the crystal may re- 
sult if these atoms have a certain form, or if in certain axes 
they attract each other differently. But im bodies capable of 
imbibition, an atom of one substance is not set upon another 
atom of the same substance, but atoms of water come between; 
atoms of water, which are not united with an atom of solid 
substance, so as to form a compound atom, as in the water of 


THEORY OF THE CELLS. 207 


crystallization, but which exist in some other unknown manner 
between the atoms of solid substance. It is not possible, 
therefore, to determine whether that part of the crystal which 
is first formed must have an angular figure or not. 

An ordinary crystal consists of a number of lamin; when 
so small as to be but just discernible, it has the form which 
the whole crystal afterwards exhibits, at least as far as regards 
the angles; we must therefore suppose that the first layer is 
formed around a very small corpuscle, which is of the same 
shape as the subsequent crystal. We will call this the pri- 
mitive corpuscle. It is doubtful what may be the shape of 
this corpuscle in the crystals which are capable of imbibition. 
The first layer, then, is formed around the corpuscle in the 
way mentioned; it grows by intussusception, and thus forms a 
hollow, round or oval vesicle, to the inner surface of which the 
primitive corpuscle adheres. As all the new molecules that are 
being deposited may be placed in this layer without any altera- 
tion being required in the law which regulates the coalescence 
of the molecules during crystallization, we must conclude that 
it remains the only layer, and becomes greatly expanded, so as 
to represent all the layers of an ordinary crystal. It is, how- 
ever, a question whether there may not exist some reasons why 
several layers can be formed. We can certainly conceive such 
to be the case. The quantity of the solid substance that must 
crystallize in a given time, depends upon the concentration of 
the fluid; the number of molecules that may, im accordance with 
the law already mentioned, be deposited in the layer in a given 
time depends upon the quantity of the solution which can 
penetrate the membrane by imbibition during that time. If 
in consequence of the concentration of the fluid there must be 
more precipitated in the time than can penetrate the mem- 
brane, it can only be deposited as a new layer on the outer 
surface of the vesicle. When this second layer is formed, the 
new molecules are deposited in it, and it rapidly becomes ex- 
panded into a vesicle, on the inner surface of which the first 
vesicle lies with its primitive corpuscle. The first vesicle now 
either does not grow at all, or at any rate much more slowly, 
and then only when the endosmosis into the cavity of the 
second vesicle proceeds so rapidly that all that might be pre- 
cipitated while passing through it, is not deposited. The second 


208 THEORY OF THE CELLS. 


vesicle, when it is developed at all, must needs be developed 
relatively with more rapidity than the first; for as the solution 
is in the most concentrated state at the beginning, the necessity 
for the formation of a second layer then occurs sooner ; but 
when it is formed, the concentration of the fluid is diminished, 
and this necessity occurs either later or not at all. It is pos- 
sible, however, that even a third, or fourth, and more, may be 
formed ; but the outermost layer must always be relatively the 
most vigorously developed ; for when the concentration of the 
solution is only so strong, that all that must be deposited in a 
certain time, can be deposited in the outermost layer, it is all 
applied to the increase of this layer. 

Such, then, would be the phenomena under which substances 
capable of imbibition would probably crystallize, if they did so 
at all. I say probably, for our incomplete knowledge of crys- 
tallization and the faculty of imbibition, does not as yet admit 
of our saying anything positively @ priori. It is, however, 
obvious that these are the principal phenomena attending the 
formation of cells. They consist always of substance capable 
of imbibition ; the first part formed is a small corpuscle, not 
angular (nucleolus), around this a lamina is deposited (nucleus), 
which advances rapidly in its growth, until a second lamina 
(cell) is formed around it. This second now grows more quickly 
and expands into a vesicle, as indeed often happens with the 
first layer. In some rarer instances only one layer is formed ; 
in others, again, there are three. The only other difference in 
the formation of cells is, that the separate layers do not con- 
sist of the same chemical substance, while a common crystal 
is always composed of one material. In instituting a com- 
parison, therefore, between the formation of cells and crystal- 
lization, the above-mentioned differences in form, structure, 
and mode of growth fall altogether to the ground. If crystals 
were formed from the same substance as cells, they would pro- 
bably, in ¢hese respects, be subject to the same conditions as 
the cells. Meanwhile the metabolic phenomena, which are 
entirely absent in crystals, still indicate essential distinctions. 

Should this important difference between the mode of for- 
mation of cells and crystals lead us to deny all intimate con- 
nexion of the two processes, the comparison of the two may 
serve at least to give a clear representation of the cell-life. 


THEORY OF THE CELLS. 209 


The following may be conceived to be the state of the matter : 
the material of which the cells are composed is capable of 


producing chemical changes in the substance with which it is | 
in contact, just as the well-known preparation of platinum | 


converts alcohol into acetic acid. This power is possessed by 
every part of the cell. Now, if the cytoblastema be so changed 
by a cell already formed, that a substance is produced which 
cannot become attached to that cell, it immediately crystallizes 
as the central nucleolus of a new cell. And then this con- 
verts the cytoblastema in the same manner. A portion of that 
which is converted may remain in the cytoblastema in solution, 
or may crystallize as the commencement of new cells ; another 
portion, the cell-substance, crystallizes around the central cor- 
puscle. The cell-substance is either soluble in the cytoblastema, 
and crystallizes from it, so soon as the latter becomes saturated 
with it; or else it is insoluble, and crystallizes at the time of 
its formation, according to the laws of crystallization of bodies 
capable of imbibition mentioned above, forming in this manner 
one or more layers around the central corpuscle, and so on. 
If we conceive the above to represent the mode of the formation 
of cells, we regard the plastic power of the cells as identical 
with the power by which crystals grow. According to the 
foregoing description of the crystallization of bodies capable of 
imbibition, the most important plastic phenomena of the cells 
are certainly satisfactorily explained. But let us see if this 
comparison agrees with all the characteristics of the plastic 
power of the cells. (See above, p. 194 et seq.) 

The attractive power of the cells does not always operate 
symmetrically ; the deposition of new molecules may be more 
vigorous in particular spots, and thus produce a change in the 
form of the cell. This is quite analogous to what happens in 
crystals ; for although in them an angle is never altered, there 
may be much more material deposited on some surfaces than 
on others ; and thus, for instance, a quadrilateral prism may be 
formed out of a cube. In this case new layers are deposited on 
one, or on two opposite sides of a cube. Now, if one layer in 
cells represent a number of layers in a common crystal, it may 
be easily perceived that instead of several new layers being 
formed on two opposite surfaces of a cell, the one layer would 
grow more at those spots, and thus a round cell would be elon- 

14 


} 


210 THEORY OF THE CELLS. 


gated into a fibre ; and so with the other changes of form. Divi- 
sion of the cells can have no analogue in common crystals, 
because that which is once deposited is incapable of any further 
change. But this phenomenon may be made to accord with the 
representation of crystals capable of imbibition, just as well as 
the coalescence of numerous cells in the manner described at 
page 184 does. And if we ascribe to a layer of a crystal capa- 
ble of imbibition the power of producing chemical changes in 
organic substances, we can very well understand also the origin 
of secondary deposits on its inner surface as they occur im cells. 
For if, in accordance with the laws of crystallization, the lamina 
has become expanded into a vesicle, and its cavity has become 
filled by imbibition with a solution of organic substance, there 
may be materials formed by means of the converting influence 
of the lamina, which cannot any longer be held in solution. 
These may, then, either crystallize within the vesicle, as new 
erystals capable of imbibition under the form of cells; or if 
they are allied to the substance of the vesicle, they may so 
crystallize as to form part of the system of the vesicle itself: 
the latter may occur in two ways, the new matters may be 
applied to the increase of the vesicle, or they may form new 
layers on its inner surface from the same cause which led to 
the first formation of the vesicle itself as a layer. In the cells 
of plants these secondary deposits have a spiral arrangement. 
This is a very important fact, though the laws of crystallization 
do not seem to account for the absolute necessity of it. If, 
however, it could be mathematically proved from the laws of 
the crystallization of morganic bodies, that under the altered 
circumstances in which bodies capable of imbibition are placed, 
these deposits must be arranged in spiral forms, it might be 
asserted without hesitation that the plastic power of cells and 
the fundamental powers of crystals are identical. 

We come now, however, to some peculiarities in the plastic 
power of cells, to which we might, at first sight, scarcely expect 
to find anything analogous in crystals. The attractive power 
of the cells manifests a certain degree of election in its opera- 
tion ; it does not attract every substance present im the cyto- 
blastema, but only particular ones; and here a muscle-cell, 
there a fat-cell, is generated from the same fluid, the blood. 
Yet crystals afford us an example of a precisely similar pheno- 


THEORY OF THE CELLS. 211 


menon, and one which has already been frequently adduced as 
analogous to assimilation. If a crystal of nitre be placed in 
a solution of nitre and sulphate of soda, only the nitre crystal- 
lizes; when a crystal of sulphate of soda is put in, only the 
sulphate of soda crystallizes. Here, therefore, there occurs just 
the same selection of the substance to be attracted. 

We observed another law attending the development of the 
plastic phenomena in the cells, viz. that a more concentrated 
solution is requisite for the first formation of a cell than for 
its growth when already formed, a law upon which the differ- 
ence between organized and unorganized tissues is based. In 
ordinary crystallization the solution must be more than satu- 
rated for the process to begin. But when it is over, there 
remains a mother lye, according to Thénard, which is no 
longer saturated at the same temperature. This phenomenon 
accords precisely with the cells; it shows that a more con- 
centrated solution is requisite for the commencement of 
crystallization than for the increase of a crystal already 
formed. The fact has indeed been disputed by Thomson ; 
but if, in the undisputed experiment quoted above, the crystal 
of sulphate of soda attracts the dissolved sulphate of soda 
rather than the dissolved nitre, and vice versd, the crystal of 
nitre attracts the dissolved nitre more than the dissolved sul- 
phate of soda, it follows that a crystal does attract a salt held 
in solution, because the experiment proves that there are de- 
grees of this attraction. But if there be such an attraction 
exerted by a crystal, then the introduction of a crystal into a 
solution of a salt, affords an efficient cause for the deposition 
of this salt, which does not exist when no crystal is introduced. 
The solution must therefore be more concentrated in the latter 
case than in the former, though the difference be so slight as 
not to be demonstrable by experiment. It would not, how- 
ever, be superfluous to repeat the experiments. In the in- 
stance of crystals capable of imbibition, this difference may be 
considerably augmented, since the attraction of molecules may 
increase perhaps considerably by the penetrating of the solution 
between those already deposited. 

We see then how all the plastic phenomena in the cells 
may be compared with phenomena which, in accordance with 
the ordinary laws of crystallization, would probably appear if 


212 THEORY OF THE CELLS. 


bodies capable of imbibition could be brought to crystallize. 
So long as the object of such a comparison were merely to 
render the representation of the process by which cells are 
formed more clear, there could not be much urged against it ; 
it involves nothing hypothetical, since it contains no explana- 
tion ; no assertion is made that the fundamental power of the 
cells really has something in common with the power by which 
crystals are formed. We have, indeed, compared the growth 
of organisms with crystallization, in so far as in both cases 
solid substances are deposited from a fluid, but we have not 
therefore asserted the identity of the fundamental powers. So 
far we have not advanced beyond the data, beyond a certain 
simple mode of representing the facts. 

The question is, however, whether the exact accordance of 
the phenomena would not authorize us to go further. If the 
formation and growth of the elementary particles of organisms 
have nothing more in common with crystallization than merely 
the deposition of solid substances from out of a fluid, there is 
certainly no reason for assuming any more intimate connexion 
of the two processes. But we have seen, first, that the laws 
which regulate the deposition of the molecules forming the 
elementary particles of organisms are the same for all ele- 
mentary parts; that there is a common principle in the deve- 
lopment of all elementary parts, namely, that of the formation 
of cells; it was then shown that the power which induced the 
attachment of the new molecules did not reside in the entire 
organism, but in the separate elementary particles (this we 
called the plastic power of the cells); lastly, it was shown that 
the laws, according to which the new molecules combine to 
form cells, are (so far as our incomplete knowledge of the laws 
of crystallization admits of our anticipating their probability) 
the same as those by which substances capable of imbibition 
would crystallize. Now the celis do, in fact, consist only of 
material capable of imbibition ; should we not then be justi- 
fied in putting forth the proposition, that the formation of the 
elementary parts of organisms is nothing but a crystallization 
of substance capable of imbibition, and the organism nothing 
but an aggregate of such crystals capable of imbibition ? 

To advance so important a point as absolutely true, would 
certainly need the clearest proof; but it cannot be said that 


THEORY OF THE CELLS. 213 


even the premises which have been set forth have in all points 
the requisite force. or too little is still known of the cause 
of crystallization to predict with safety (as was attempted above) 
what would follow if a substance capable of imbibition were to 
crystallize. And if these premises were allowed, there are two 
other points which must be proved in order to establish the 
proposition in question: 1. That the metabolic phenomena of 
the cells, which have not been referred to in the foregoing 
argument, are as much the necessary consequence of the faculty 
of imbibition, or of some other peculiarity of the substance of 
cells, as the plastic phenomena are. 2. That if a number of 
crystals capable of imbibition are formed, they must combine 
according to certain laws so as to form a systematic whole, 
similar to an organism. Both these points must be clearly 
proved, in order to establish the truth of the foregoing view. 
But it is otherwise if this view be adduced merely as an hypo- 
thesis, which may serve as a guide for new investigations. In 
such case the inferences are sufficiently probable to justify 
such an hypothesis, if only the two points just mentioned can 
be shown to accord with it. 

With reference to the first of these points, it would certainly 
be impossible, in our ignorance as to the cause of chemical 
phenomena in general, to prove that a crystal capable of im- 
bibition must produce chemical changes in substances sur- 
rounding it; but then we could not infer, from the manner 
in which spongy platinum is formed, that it would act so 
peculiarly upon oxygen and hydrogen. But in order to 
render this view tenable as a possible hypothesis, it is only 
necessary to see that it may be a consequence. It cannot be 
denied that it may: there are several reasons for it, though 
they certainly are but weak, For imstance, since all cells 
possess this metabolic power, it is more likely to depend on a 
certain position of the molecules, which in all probability is 
essentially the same in all cells, than on the chemical com- 
bination of the molecules, which is very different in different 
cells. The presence, too, of different substances on the inner 
and the outer surface of the cell-membrane (see above, page 
199) in some measure implies that a certain direction of the 
axes of the atoms may be essential to the metabolic pheno- 
mena of the cells. I think, therefore, that the cause of the 


214 THEORY OF THE CELLS. 


metabolic phenomena resides in that definite mode of arrange- 
ment of the molecules which occurs in crystals, combined with 
the capacity which the solution has to penetrate between these 
regularly deposited molecules (by means of which, presuming 
the molecules to possess polarity, a sort of galvanic pile will 
be formed), and that the same phenomena would be observed 
in an ordinary crystal, if it could be rendered capable of imhbi- 
bition. And then perhaps the differences of quality in the 
metabolic phenomena depend upon their chemical composition. 

In order to render tenable the hypothesis contained in the 
second point, it is merely necessary to show that crystals capable 
of imbibition can unite with one another according to certain 
laws. If at their first formation all crystals were isolated, if 
they held no relation whatever to each other, the view would 
leave entirely unexplained how the elementary parts of or- 
ganisms, that is, the crystals in question, become united to 
form a whole. It is therefore necessary to show that crystals 
do unite with each other according to certain laws, in order to 
perceive, at least, the possibility of their uniting also to form 
an organism, without the need of any further combining 
power. But there are many crystals in which a union of this 
kind, according to certain laws, is indisputable; imdeed they 
often form a whole, so like an organism in its entire form, 
that groups of crystals are known in common life by the names 
of flowers, trees, &c. I need only refer to the ice-flowers on the 
windows, or to the lead-tree, &e. In such instances a number 
of crystals arrange themselves in groups around others, which 
form an axis. If we consider the contact of each crystal with 
the surrounding fluid to be an indispensable condition to the 
growth of crystals which are not capable of imbibition, but that 
those which are capable of imbibition, in which the solution can 
penetrate whole layers of crystals, do not require this condition, 
we perceive that the similarity between organisms and these 
aggregations of crystals is as great as could be expected with 
such difference of substance. As most cells require for the 
production of their metabolic phenomena, not only their pe- 
culiar nutrient fluid, but also the access of oxygen and the 
power of exhaling carbonic acid, or vice versd ; so, on the other 
hand, organisms in which there is no circulation of respiratory 
fluid, or in which at least it is not sufficient, must be developed 


THEORY OF THE CELLS. 215 


in such a way as to present as extensive a surface as possible 
to the atmospheric air. ‘This is the condition of plants, which 
require for their growth that the individual cells should come 
into contact with the surrounding medium in a similar manner, 
if not in the same degree as occurs in a crystal tree, and in 
them indeed the cells unite into a whole organism in a form 
much resembling a crystal tree. But in animals the circulation 
renders the contact of the individual cells with the surrounding 
medium superfluous, and they may have more compact forms, 
even though the laws by which the cells arrange themselves 
are essentially the same. 

The view then that organisms are nothing but the form 
under which substances capable of imbibition crystallize, ap- 
pears to be compatible with the most important phenomena of 
organic life, and may be so far admitted, that it is a possible 
hypothesis, or attempt towards an explanation of these pheno- 
mena. It involves very much that is uncertain and paradoxical, 
but I have developed it in detail, because it may serve as a 
guide for new investigations. For even if no relation between 
crystallization and the growth of organisms be admitted in 
principle, this view has the advantage of affording a distinct 
representation of the organic processes ; an indispensable re- 
quisite for the institution of new inquiries in a systematic © 
manner, or for testing by the discovery of new facts a mode 


of explanation which harmonizes with phenomena already 
known. 


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SUPPLEMENT 


(REFERRED TO AT P. 46) 


ON THE SIGNIFICATION OF THE GERMINAL VESICLE. 


Wuen treating of the different parts of the ovum, in the 
foregoing work, it was found impossible to give a positive 
solution to the question as to whether the germ-vesicle was a 
young cell or the nucleus of the yelk-cell. Most of the facts 
before us were in favour of the latter view ; but if this were the 
correct one, the yelk-cell ought to be developed around the 
previously existing vesicle in such manner, that it in the 
first instance closely encompassed the latter, and afterwards 
became gradually expanded. This decisive observation was 
wanting, and the researches communicated by R. Wagner, in 
his ‘ Prodromus,’ rather tended to show that, in the formation 
of the ovum around the germinal vesicle, the membrane was 
not formed immediately around the vesicle, but that it inclosed 
at the same time a quantity of the granular mass in which the 
germ-vesicle lies. I was not at that time acquainted with a 
work of Wagner’s, which contained the facts necessary to a 
solution of the question, viz. his ‘ Beitrage zur Geschichte 
der Zeugung und Entwickelung., Erster Beitrag :’ from the 
‘ Mathematisch-physikalischen Klasse der K6nigl. Baierschen 
Acad. der Wissenschaften in Munchen.’ Speaking of the 
ovaries of insects, Wagner says, at page 45 :—* At the spot 
where the oviduct widens, the granular mass, which resembles 
the vitellme mass, becomes more plentiful; the separate germ- 
‘vesicles seem to be imbedded in it. I have so represented it 
in the ‘ Prodromus,’ fig. 18. Lately, however, it has appeared 
to me, as though the germ-vesicles with their germinal spots 
were actually already surrounded by a chorion and a perfectly 
pellucid yelk.” The accompanying illustration from Agrion 
virgo exhibits clearly how that which Wagner calls chorion, 


218 SUPPLEMENT. 


or the cell-membrane of the yelk-cell, closely encompasses the 
germ-vesicle at first and then gradually expands, while between 
it and the vesicle a transparent fluid collects; in which, ata 
later period, a turbidness commences, occurring first in the 
neighbourhood of the germ-vesicle. Wagner had thus dis- 
covered in the course of his observations that the details of 
the process were just what must have been expected according 
to the theory of the unity of the principle of development for 
all elementary particles of the organism. That the germ- 
vesicle is the nucleus of the yelk-cell appears to me therefore 
to be scarcely dubitable. The illustration given by Wagner 
also shows that the germinal spot is first developed, then the 
germ-vesicle around it, and around this again the yelk-cell. It 
is not surprising that granulous contents may form within 
the germ-vesicle at a subsequent period, since the same thing 
occurs in the indubitable nucleus of the adipose cells of the 
fish, and the formation of the cell is probably nothing more 
than a repetition of that same process around the nucleus, by 
means of which the nucleus was originally formed around the 
nucleolus. 


REMARKS 


UFON A STATEMENT PUT FORTH BY PROFESSOR VALENTIN, 
RESPECTING PREVIOUS RESEARCHES ON THE SUBJECT OF 
THIS WORK. 


Arter I had finished this Treatise, I received the first part 
of Wagner’s ‘Lehrbuch der Physiologie! Leipzig, 1839; 
which was just then issuing from the press, and which con- 
tained (at page 182) an outline of the development of the 
animal tissues, communicated by Professor Valentm. The 
author introduces the subject with some historical remarks, 
in which he represents my researches as giving an essen- 
tial completeness to the analogies between animal tissues 
and vegetable cells which had been previously pointed out, 
more particularly by himself. There are very many ways 
of drawing a comparison between two objects, and simi- 
litudes may be discovered which are opposed to the whole 
internal construction of the things in which they are observed. 
Everything, therefore, depends upon the sort of comparison 
drawn. If Valentin’s historical representation be justified, the 
idea of a comparison, similar in its kind to that on which my 
researches are based, must have a previous existence in his 
earlier investigations. I have endeavoured to analyse the 
fundamental idea of my investigation in the commencement 
of the Third Section of this treatise ; it was this—that one 
common principle of development forms the basis of all the 
elementary particles of organisms. It origmated in a com- 
parison being drawn between a cartilage-cell and a vegetable 
cell, in such sense, that the molecules are united together for 
the formation of both of them, in accordance with similar laws, 
since in both instances a nucleolus is first fermed ; around this 


’ Rudolph Wagner’s Elements of Physiology, translated by R. Willis, M.p., p. 214. 


220 REMARKS UPON A STATEMENT 


a nucleus, and around this again a cell. The accordance in the 
mode of development of two so different elementary particles, 
first led to the deduction of the principle of a similar mode of 
formation for all elementary particles, and then to its proof by 
observation. Therefore, what we have to decide is, first, 
whether the idea of comparing an animal elementary structure 
with a vegetable cell, with reference to a similar mode of de- 
velopment, does occur in Valentin’s earlier observations ; and, 
secondly, whether Valentin has recognised the principle which 
is contained in the similar mode of development of two ele- 
mentary particles which, in a physiological sense, are very dis- 
similar. In my preface I have given a brief historical sketch 
of the subject from my own point of view, and Valentin’s 
remarks do not convince me of the necessity of making any 
alteration in it. Impartiality, however, requires that Valentin’s 
representation should follow this statement, and I therefore 
append the passages cited by him, word for word, from his 
works : 


“In my first histogenetic researches, I observed certain pecu- 
liar granules lying in a transparent gelatinous substance, as the 
primordial matter of all the tissues. I pointed out the difference 
between these granules in the serous and mucous layers, at the 
period of the earliest separation of the layers from one another. 
In the vascular layer I found large globules or cells, which, in 

3 2) 
respect to their form and juxtaposition, I compared, as early as 
the year 1835, with vegetable cellular tissue. (Entwickelungs- 
geschichte, 287. The vascular layer seems to be composed of large 
globules having a mean diameter of 0°001013 Paris inch, which are 
perfectly transparent in their interior, and so closely crowded toge- 


ther, that they are flattened against one another at many of their points of 
contact, and assume an hexagonal form like the cellular tissue of plants.) 


I also first directed attention to the resemblance in form of the 
cartilages in which ossification was commencing, and particularly 
(from observations made in conjunction with Purkinje) of the 
branchial cartilage of the tadpole to the vegetable cellular tissue. 
(Ib. 209-10. The cartilages of the labyrinth present a variety of form 
whilst passing through the process of ossification, which differs very 
essentially from most of the other cartilages of the body, which will be 
described at greater length presently. In place of the ordinary car- 
tilage-corpuscle, they contain large bodies which are not so well defined 
in form, most of them furnished with linear boundaries, being roundish, 


Se 


PUT FORTH BY VALENTIN. 221 


semilunar, tetrahedral, or polyhedral in shape, with a mean diameter 
of from 0°000405, to 0°000650 Paris inch. But so soon as they 
ossify, the calcifying portion, or that which is already ossified, consists 
of a tissue of beautiful six-sided prisms (Balken), closely resembling 
vegetable cellular tissue, upon and within which are small granules of 
a round figure, with a diameter of about 0°000152 Paris inch. The last 
described form, was observed both by Purkinje and myself long since 
in the cartilages of the tadpole also, especially in the branchial arches.) 
I described the round celis of the globules with their interposed 


cellular substance from the chorda dorsalis of young embryos. 
(Ib. 157. Although the external appearance of the chorda dorsalis 
clearly presents a certain resemblance to a cartilage, the microscopical 
investigation of its structure most distinctly disproves similarity. In 
every instance in which it is present, it consists of an external, symme- 
trical, perfectly transparent envelope and globules of variable size, but 
always very numerous, and lying closely packed together. A gelatinous 
and perfectly transparent mass occupies the interspaces left between 
them. These globules are largest in fishes and amphibia, smaller in 
birds, and smallest in mammalia.”” In the second passage, which 
Valentin cites on this point (Repertor. i, 187), the researches 
of J. Miiller, which I have noticed at page 7 in this treatise, 
are referred to and quoted, the following also is from the same 
source :—‘“‘ which (chorda dorsalis) the reporter (Valentin) has also 
observed in fcetal pigs of eight lines in length, in the form of a thick 
cord lying within the cartilaginous vertebree, its internal structure, in 
the embryos of mammalia, birds, and amphibia, being, according to 
his observations, essentially similar to the permanent analogous forma- - 
tions of the cartilaginous fishes.) Soon after this J. Miiller, from 


his own independent investigations, gave a more detailed ex- 
planation of the cells in the spinal cord of fishes (Myzxinoiden, 
74, &c.) In the epithelia, which Purkinje and Raschkow 
(Meletem. c. mammal. dent. evol. 12), as well as I (Nov. act. 


ac. N. C. vol. xvii, p. 1. 96)——These (the tuft-like groups of the 
choroid plexus) do not lie free, but they, as well as the connecting 
granulous membrane, are covered with a very delicate and transparent 
epithelium, the separate globules of which have the most regular six- 
sided cell-border, and are perfectly colourless and transparent. Hach 
of them, however, contains, in the mass in its interior, a dark round 
nucleus, or formation, which reminds the observer of the nucleus oc- 
curring in the cells of the epidermis, the pistil, &e., in the vegetable 
kingdom. In man, whose choroid plexus exhibits a more blackish or 
dark colour even to the naked eye, the epithelium itself has a similar 
formation to that just described, but the centre of each cell contains 
in its exterior a round pigment-globule, corresponding to the central 
point of the situation of the nucleus in its interior. Similar pigment- 
globules exist in most birds, but not being so regularly deposited, it is 


222 REMARKS UPON A STATEMENT 


more difficult to detect the cell-shaped and more rounded globules, 
although they are quite as certainly present. When the object has 
not been at all damaged, the cells, and especially the pigment-globules 
adhering to the outside, exhibit an arrangement like that of the vege- 
table cells in general, and particularly in the earliest stages in the 
formation of the leaf, that is, a disposition corresponding to spiral lines 
projected on the surface in accordance with the strictest rules) 
——compared to the cellular tissue of plants, I chose, expressly 
(l.c. 77. Each of these globules (ganglion-globules), wherever ob- 
served, has an external, more or less distinct, areolar tissue-like envelope, 
and contains a parenchymatous mass proper to itself, an independent 
nucleus or kernel (nucleus oder Kern), which again encloses a 
second roundish, transparent nucleus)——on account of this re- 


semblance in form, the uniform appellation of the nucleus 
(Kernes), just as I afterwards described the nucleolus which was 


observed by me. (Repertor. i, 143. In every cell without exception 
there is a somewhat smaller and more compact nucleus of a round or 
oval form. It usually occupies the centre of each cell, consists of a 
minutely granulous substance, but encloses a well-defined, round cor- 
puscle, which thus forms a sort of second nucleus within it.) Jn the 


study of the epithelia, prosecuted particularly by Henle and 
myself, there was no want of analogies with vegetable cellular 
tissue, the individuality of the cell-parietes was also distinctly 


demonstrated. (Ib. 284. Roundish, hexagonal, flat, aud tolerably 
thin cells lie (in the external skin of the proteus) close upon one 
another, disposed in regular arrangement, and always connected 
together with their lateral edges and angles in mutual correspondence. 
The interior of these delicate bodies is filled by a granulous or yel- 
lowish mass, which represents a sort of nucleus. But the separate 
granules of this nucleus, however closely they may he together, may 
be accurately distinguished from one another. With a very strong 
magnifying power, each one of these granules may be seen to be more 
transparent in its centre than it is in its periphery. It may then 
also be most distinctly ascertained, that the somewhat delicate parietes 
of each cell are perfectly isolated from the central cavity. No trace 
of granules or fibres can be observed on the walls themselves ; there is 
merely a clear, transparent, vitreous, and homogeneous mass.) J had 
also remarked that the nuclet (pigment-vesicles) were the parts 
first formed in the pigment of the choroid coat (Entwickelungs- 
geschichte, 194. The following is the mode in which, according to 
my observations, the stratum of pigment is formed in man, mammalia, 
and birds; separate, round, colourless, and transparent corpuscles are 
first deposited upon the internal surface of the substance they are to 
cover, in the earliest period (up to the tenth week) these corpuscles in 
the human subject measure from 0:000355 to 0:000405 Paris inch in 
diameter. They are the future pigment-corpuscles or pigment-vesicles. 


a een 


ant 


PUT FORTH BY VALENTIN. 223 


Pigment-globules of a black colour are soon, however, developed on 
their periphery, so that the corpuscles or vesicles just mentioned are 
transparent in their centre when they have ceased to be so, and have 
become dark on their circumference. It is plain that von Ammon and 
R. Wagner have seen this condition as well as myself. The globules 
are so small from the commencement, that they ..... This process 
of deposition of the black-coloured globules upon the pigment-cor- 
puscles goes on afterwards continuously, and to such an extent that 
the latter are enveloped and covered on all sides by them, and are only 
rendered visible when the globules are removed by pressure or wasbing.); 
and I compared the pigment-cells with the cellular tissue of 


plants. (Repertor. ii, 245. The pigment here (in the choroid) has 
the same character which it has in most other parts of the body, that 
is, a round, clear, transparent, and colourless nucleus, or the pigment- 
molecules lie closely crowded together around a pigment-vesicle. These 
heaps of pigment composed of pigment-vesicles, and the molecules of 
pigment deposited around them, are extended out sidewise, and in man, 
the dog, the rabbit, the horse, the ox, and such lke, form unequal 
pentagons or hexagons, which are placed close together in a similar 
manner to the cells of the parenchymatous cellular tissue of plants. 
Langenbeck de retina, 38.) Schwann gave an essential complete- 
ness to these analogies, by showing that the gelatinous primordial 
mass of the tissues was composed of cells, that the bodies im- 
bedded init are nuclei, and that these and the cells often exhibit 
analogous laws of development. (Froriep’s Notizen, 1838, 
Mikroskopische Untersuchungen iiber die Struktur der Thiere 
und Pflanzen, Heft 1, 1838.) As early as 1837 I had observed © 
the cells of the germinal membrane in the ovum of sepia, with 
their nuclei and nucleoli, and the areas surrounding them, and 
had communicated my researches in a letter to Breschet. Shorily 
after I became acquainted with Schwann’s first communication I 
commenced the investigation of the subject. The chief results 
of my inquiries are contained in the following communication. I 
have, at the same time, referred to the corresponding passages 
in the first part of Schwann’s treatise, which I have received 
this day.” 


I will only add that the second part also, (consisting of sheets 
8 to 18, and Plates III and IV,) therefore the whoie of the 
portion of my treatise containing the observations, had appeared 
previous to Valentin’s researches, and had been communicated to 
the Parisian Academy in the year 1838; a remark which does 
not appear altogether superfluous, since Professor Wagner has 


224 REMARKS UPON A STATEMENT, ETC. 


communicated an epitome of my observations (which I sent to 
him four weeks after he had requested it from me) in his 
Physiology, with the remark that it had arrived later than 
the observations of Valentin. Moreover, even my first com- 
munications in Froriep’s Notizen contained the fundamental 
laws for the formation of all the tissues, and the details also 
respecting by far the most of them. 


ae 
ah wee aut. 


Th. Schwang, del . 


EXPLANATION OF THE PLATES. 


WuHere no other measurement is given, the figure re- 
presents the object magnified about 450 diameters, linear 
measurement. 


PLATE I. 
Fig. 1. Parenchymatous cellular tissue, with cell-nuclei from 
an onion, magnified 290 times. 
2, Matrix of the pollen of Rhipsalis salicornoides. 
3. Do. do. 


I am indebted to the kindness of Dr. Schleiden for the last. two delineations. 


4, Cells from the chorda dorsalis of Cyprinus erythroph- 
thalmus. 


5. Cartilage from the point of a branchial ray, from the 
same. 


6. Cartilage from the middle of a branchial ray, from the 
same. 


7. Cartilage from the root of a branchial ray, from the 
same. 


8. Branchial cartilage from the larva of Rana esculenta. 


9. Cranial cartilage (ethmoid bone) from the larva of 
Pelobates fuscus. 


10. Cells from the crystalline lens of a fetal pig four 
inches long. 


11. An isolated nucleus of the cells of the crystalline lens. 


12. Cells from the crystalline lens of the same feetus, ex- 
hibiting their prolongation into the fibres of the lens. 


13. Fibres from the innermost layers of the lens of a pike. 


14, Cell from the epidermis of a species of grass. 
15 


226 

Fig. 1. 
2 
3 
4 
5 


“I 


8 and 9. Pigment-cells of different kinds and stages of . 


EXPLANATION OF THE PLATES. 


PLATE MIL 


Ovum of a goat, after Krause (Miller’s Archiv, 1837, 
Pls A; es); 


. Cells from the yelk-cavity of a mature hen’s egg. 


. Cells from the interior of an egg measuring a line and 


a half in diameter, taken from the ovary of a hen. 


. Portion of the germinal membrane of a mature hen’s 


egg before incubation, viewed from above. 


. Portion of the germinal membrane from a hen’s egg 


after sixteen hours’ incubation. It is folded in such 
a manner that the external surface or serous layer 
forms the margin. 


. Cells from the serous layer of the same germinal mem- 


brane in the neighbourhood of the area pellucida, 
after separation of the mucous layer. 


. Cells from the mucous layer of the same germinal 


membrane on the outside of the area pellucida. 


development, from the tail of the tadpole. 


10. Cells from the interior of the shaft of a fully deve- 


loped wing-feather of the raven. 


11. Earlier stages of development of the same, from the 


portion of the shaft of an immature feather which 
has not as yet become hard. 


12. Cell-nuclei, from the same, around which no cells have 


as yet formed. 


13. Flat cells splitting into fibres, from the cortex on the 


side of the shaft of a raven’s feather in progress of 
formation. 


be aos 
= Em 


2 


PLATE 


a 


Fa. 


sc. 


- 
@ 
<a 
.¢ 
~ 4 
- oe 


eS ig 
* r 


Ay 


4 


eM 


ne 


a) 


fo) | 


6. 


EXPLANATION OF THE PLATES. 227 


PLATE III. 


. From the point of a branchial cartilage of Rana 


esculenta. The lower margin of the delineation 
exhibits the natural border of the cartilage. 


. Cartilage from the ilium of a feetal pig five inches 


long, after the application of acetic acid. 


. Enamel fibres from immature teeth of a foetal pig. 
. Cells from the surface of the enamel membrane. 


. Fibres which compose the substantia propria of the 


human tooth, isolated by maceration for two days 
in dilute hydrochloric acid. 


Fibre-cells from the areolar tissue lying beneath the 
superficial muscles of the neck of a fcetal pig mea- 
suring seven inches. 


7. A more fully developed cell of areolar tissue. 


8. Cells from the gelatinous substance between the cho- 


10. 


Ly 


i: 


13. 


rion and amnion of a foetal pig seven inches long. 


. Larger and very pale cells from the areolar tissue of 


the orbital cavity of the same foetus. 


Fat-cells from the cranial cavity of the young of 
Cyprinus erythrophthalmus. 


Fibre-cells from the tendo achillis of a foetal pig three 
and a half inches long. 


From the middle coat of the aorta of a fetal pig 
measuring seven inches in length. 


Cells from the interior of the quadratus lumborum 
muscle of a foetal pig three and a half inches long. 


228 


Hie. 1. 


EXPLANATION OF THE PLATES. 


PLATE IV. 


Dorsal muscles of a foetal pig three and a half inches 
long. 


. The fibre c from the previous figure, after the applica- 


tion of acetic acid. 


. From the brachial muscles of a foetal pig seven 


inches long. 


. Primitive muscular fasciculus from the cockchafer. 


5. Muscular fasciculus from a pike. 


Las 


. A portion of the ischiatic nerve of a foetal pig mea- 


suring four inches. 


. Fasciculus of nervous fibres from the brachial plexus 


of a foetal pig four inches in length. 


. Single nervous fibres: a, from the nervus trigeminus 


of a foetal pig measuring six inches and a half; 8, 
c, d, from the nervus ischiadicus of the same. 


. Nervous fibre from the vagus of a calf. 


10. 


Ganglion-globules from the lowest ganglia of the 
sympathetic of a frog. 


Capillary vessels in the tail of the tadpole. 


. Ideal representation of the formation of the capillary 


vessels in the area pellucida of a hen’s egg. 


CES ~ 


ey 


rae 


a 


_ .Schwamn del. 


qa 7 rr 


s 
= 
* ar 
. ry, id 
+7) we an in 
i, oo 


j 
a/) | 
P h a) j 
ad lpn a LT eve to 
yA j pit 

VA) pe wg ¢ mai? De i 
li EP is : as i hae 

nm ua | 
riers 7 


CONTRIBUTIONS TO PHYTOGENESIS, 


TRANSLATED FROM THE GERMAN 


OF 


DPR M. J. SC BREE DEN, 


PROFESSOR OF BOTANY IN TITE UNIVERSITY OF JENA. 


CONTRIBUTIONS TO PHYTOGENESIS.' 


Tue general fundamental law of human reason, its unde- 
viating tendency to unity in its acquisition of knowledge, has 
always been evinced in the department which treats of or- 
ganized bodies as fully as in all other branches of science ; 
and manifold have been the endeavours to establish the ana- 
logies between the two great divisions of the animal and 
vegetable kingdoms. But eminent as the men have been who 
have devoted their attention to this subject, it cannot be 
denied that all attempts which have been hitherto made with 
this view must be regarded as entirely unsuccessful. If, in- 
deed, the fact has of late been pretty generally admitted, still 
the reason of the circumstance has not always been quite 
correctly apprehended and put forth in its full precision and. 
clearness. The cause of this, however, is, that the idea of 
individual, in the sense in which it occurs in animal nature, 
cannot in any way be applied to the vegetable world. It is 
only in the very lowest orders of plants, in some Alge and 
Fungi for instance, which consist only of a single cell, that we 
can speak of an individual in this sense. But every plant 
developed in any higher degree, is an aggregate of fully indi- 
vidualized, independent, separate beings, even the cells them- 
selves. 

Each cell leads a double life: an independent one, pertain- 
ing to its own development alone; and another incidental, in 


1 [These first appeared in Miller’s Archiv fiir Anatomie und Physiologie, Part II, 
1838. But as they have been republished with some additional notes in a collected 
edition of Schleiden’s papers, entitled ‘ Beitriige zur Botanik,’ I have made use of the 
latter work as my text; with the exception of the notes, I believe it corresponds 
precisely with the paper in Miiller’s Archiv; which, it is also right I should state, 
has been already most faithfully translated by Mr. Francis, in Taylor’s ‘ Scientific 
Memoirs,’ vol. ii, Part VIL.—TRANSLATOR. | 


232 CONTRIBUTIONS TO 


so far as it has become an integral part of a plant. It is, 
however, easy to perceive that the vital process of the indi- 
vidual cells must form the very first, absolutely indispensable 
fundamental basis, both as regards vegetable physiology and 
comparative physiology in general; and, therefore, in the very 
first instance, this question especially presents itself: how does 
this peculiar little organism, the cell, originate ? 

The great importance of the subject is the only excuse I 
can adduce for venturing at the present moment to publish the 
following remarks, feeling as I do only too well convinced 
that more extended researches can alone impart to them their 
proper scientific value. Perhaps, however, I may succeed by 
these remarks in drawing attention to this very important 
subject. 

Since no real advance in science results from the attempt 
to explain natural phenomena hypothetically, and least of all, 
where all the conditions for the erection of a tenable hypo- 
thesis, namely, guiding facts, are wanting, I may omit all 
historical introduction; for, so far as I am acquainted, no 
direct observations exist at present upon the development of 
the cells of plants. Sprengel’s pretended primitive cells have 
long since been shown to be solid granules of amylum. To 
enter upon Raspail’s work appears to me incompatible with 
the dignity of science. Whoever feels any desire to do so, 
may refer to the work itself. 

The only work connected with this subject, the highly dis- 
tinguished one by Mirbel, I shall have occasion to refer to 
subsequently, since even he does not make any allusion to the 
process of cell-formation. It is to be regretted that Meyen, 
who perhaps has studied vegetable anatomy more comprehen- 
sively than any one up to the present time, should have con- 
fined himself almost exclusively to the investigation of deve- 
loped forms, and not yet have brought the formative process 
itself in any degree within the sphere of his enquiries. I still 
have many doubts, the solution of which I had hoped to have 
found in his Physiology, but hoped in vain. 

It was Robert Brown who, with his comprehensive natural 
genius, first realized the importance of a phenomenon, which, 
although observed previously by others, had yet remained 
totally neglected. He found, in the first mstance, m a great 


PHYTOGENESIS. 233 


many of the cells in the epidermis of the Orchidee, an opaque 
spot, named by him areola, or nucleus of the cell. He subse- 
quently pursued this phenomenon in the earlier stages of the 
pollen-cells, in the young ovulum, in the tissue of the stigma, 
not only in the Orchidee, but also in many other Monocotyle- 
dons, and even in some Dicotyledons. 

As the constant presence of this areola in the cells of very 
young embryos and in the newly-formed albumen could not 
fail to strike me in my extensive investigations into the deve- 
lopment of the embryo, it was very natural that the consider- 
ation of the various modes of its occurrence should lead to the 
thought, that this nucleus of the cell must hold some close 
relation to the development of the cell itself. I consequently 
directed my attention particularly to this point, and was for- 
tunate enough to see my endeavours crowned with success. 

Before, however, I proceed to the communication of these 
observations, I must first give a somewhat more detailed 
description of the nucleus. As I have to treat of a peculiar 
and, I think, universal elementary organ of vegetables, I do 
not consider it necessary to apologise for applying a definite 
name to this body, and therefore call it Cytoblast (kuroe, 
(Aacroc) in reference to its function, which will be described 
hereafter. . 

This formation varies in its outline from oval to circular, 
according as the solid which it forms passes from the lenticular 
into the perfectly spheroidal figure. I have found the oval 
and flat cytoblasts more frequently in Monocotyledons, m the 
albumen and pollen; the globular chiefly in the Dicotyledons, 
and in the leaf, stem, articulated hairs, and similar structures ; 
no exclusive rule, however, can be laid down on this point. 

The colour of the cytoblast is in general yellowish, but it 
sometimes passes into an almost silvery white. I remarked it 
as being most transparent in the albumen of some water 
plants, in the unripe pollen, in some Orchidee, and also in the 
rudiments of the leaf of Crassula portulaca. Its excessive 
transparency renders it scarcely perceptible in the spores of 
some Helvelloids. It is coloured by iodine, according to its 
various modifications, from a pale yellow to the darkest brown. 

It varies considerably in size. It is in general largest in 
Monocotyledons, and in the albumen; and smallest in Dico- 


234 CONTRIBUTIONS TO 


tyledons, mm the leaf, stem, and their metamorphosed parts. 
The largest which I have seen measured 0:0022 Paris inch in 
diameter (in Fritillaria pyrenaica) ; the smallest, im the em- 
bryonal extremity of the pollen-tube of Linum pallescens, from 
0:00009 to 0-0001 Paris inch. In the albumen of Abies excelsa 
I found the average of several admeasurements of examples, 
which appeared of equal size, to be 0:00034:-0-00059 -0:00079. 
In the young leaves of Crussula portulaca, 0-0003; and in the 
albumen of Pimelea drupacea, 0:00095-0:001055. Little im- 
portance, however, can, on the whole, be attached to these 
admeasurements, since they increase and diminish, and we 
cannot determine in what period of its existence the cytoblast 
may be at the time. 

Its internal structure is in general granulous, without, how- 
ever, the granules, of which it consists, being very clearly dis- 
tinct from each other. Its consistence is very variable, from 
such a degree of softness as that it almost dissolves in water, 
to a firmness which bears a considerable pressure of the com- 
pressorium without alteration of form. The more recent its 
formation, the softer it is; and this also applies to cases in 
which its existence is merely transitory. It is denser and more 
sharply defined when it endures throughout the whole vital 
process of the plant as a permanent tissue, as in the Orchidee. 

These peculiarities have been more or less fully described 
by R. Brown (Organs and Mode of Fecundation in Orchideze 
and Asclepiadez ; Linn. Trans. 1833, p. 710), and recently by 
Meyen (Physiologie, &c., Bd. I, p. 207). A phenomenon, how- 
ever, has escaped both of these most acute observers, which I 
am notwithstanding disposed to regard as one of the most 
essential. In very large and beautifully developed cytoblasts, 
for example, in the recently formed albumen of Phormium 
tenax and Chamedorea schiedeana (pl. I, fig. 5), there is ob- 
served (whether sunk in the interior or on its surface, is not 
yet clear to me) a small, sharply defined body, which, judging 
from the shadow that it casts, appears to represent a thick 
ring, or a thick-walled hollow globule. In examples which are 
not so well developed, only the external sharply defined circle of 
this ring can be observed, and in its centre a dark point; for 
example, in the stipes of the embryo of Limnanthes Douglasii, 
Orchis latifolia (pl. I, fig. 21), Pimelea drupacea (figs. 14, 15). 


PHYTOGENESIS. 235 


In still smaller cytoblasts it appears only as a sharply cir- 
cumscribed spot; this is most frequently the case, as in the 
pollen of Richardia ethiopica, in the young embryo of Linum 
pallescens, and in almost all Orchidee (fig. 16); or, lastly, 
only a remarkable small dark point is observed. I have not, 
as yet, succeeded in discovering it in the very smallest and 
most transitory cytoblasts (in the leaves of Dicotyledons for 
instance). I have also found two in some very rare cases, but 
they occurred as exceptions to the general rule, and always 
where the majority exhibited the simple nucleus ; for example, 
in Chamedorea schiedeana (figs. 6, 7), Secale cereale, Pimelea 
drupacea (fig. 14); m the two latter I have sometimes found 
even three (fig. 15). The observations I have made upon all 
plants in which it was possible to trace the entire process of 
formation completely, lead to the conclusion, that these small 
bodies are formed earlier than the cytoblast (pl. I, figs. 1, 2); 
and I am almost inclined to conjecture that they are not alto- 
gether unallied to the nuclei which Fritsche has shown to 
exist in starch, and may probably indeed be identical with 
them.' The size of this corpuscle also varies considerably, 
from the extent of half the diameter of the cytoblast to the 
most minute point, whose size could not be measured in con- 
sequence of the thread in the diaphragm of the microscope’ 
exceeding it so much in thickness. In the albumen of Adies 
excelsa I found it to average from 0:000045-0:000095 Paris 
inch ; in Pimelea drupacea, from 0:00029-0:0003. Sometimes 
it appears darker, at others brighter, than the remaining mass 
of the cytoblasts. In general it has more consistency than 
the rest of the cytoblast, and continues sharply defined after 
that has been changed by pressure into an amorphous mass, as 
in Pimelea drupacea for example. 

There is a second point, on which I must say a few words, 
in order to be enabled to express myself more briefly hereafter 
without being unintelligible, which relates to the different 
inorganic substances that occur during the vital process of 
plants, and pertain to the series of starch and woody fibre. I 
make no pretensions whatever to a complete enumeration of all 


' More accurate investigation of the structure of the starch granules lias shown 
this supposition to be quite untenable. 


236 CONTRIBUTIONS TO 


the substances which differ in a chemical sense; and just as 
little do I require that chemists should approve all my terms 
and characteristics (independent of this, perfection at the pre- 
sent time would be an impracticable task); I shall merely 
notice in a few words the most important modifications, their 
consequence and signification in the course of the develop- 
ment of vegetable organization, in order to avoid repetitions 
in future. 

In the plant starch appears almost to take the place of 
animal fat. It is superfluous nutritive material, which is de- 
posited for future use; and we therefore usually find it in 
places where a new formative process is to commence after a 
short repose, or where a too luxuriant life has generated a 
superabundance of nutritive material. It has of late been the 
subject of such deep research that it is unnecessary for me to 
enter upon it more fully; I will merely refer the reader to 
the most recent and practical summary of the results in 
Meyen’s Physiologie, Bd. I, p. 190, &c. 

The starch is sometimes supplanted by a semi-granulous 
substance ; for mstance, in pollen, the albumen of some plants, 
and frequently im the cells of the leaf, as matrix of the 
chlorophylle. It is chiefly distinguished by its occurrence in 
irregular, granulous forms, which have no internal structure, 
aud from its beg coloured a brownish-yellow or brown by 
tincture of iodine. This substance, which I shall call mucus, 
is probably identical with that of which the cytoblasts are com- 
posed, and with the small granules in gum, which I shall pre- 
sently mention. Meyen has already remarked the probability 
of the first supposition (Physiologie, Bd. I, p. 208). 

But when the starch is to be employed in new formations, 
it becomes dissolved, in a manner as yet quite unknown in 
chemistry, into sugar or gum, the latter sometimes appearing to 
pass into the former, or vice versd. The sugar appears in the 
form of a perfectly transparent fluid, which is almost as clear 
as water, is not rendered turbid by alcohol, and receives from 
tincture of iodine only so much colour as corresponds to the 
strength or weakness of the solution of the reagent. 

The gum appears as a somewhat yellowish, more consistent, 
and less transparent fluid, which is coagulated into granules by 
tincture of iodine, assuming a pale yellow permanent colour. 


PHYTOGENESIS. 237 


In the further progress of organization (in which process the 
gum is always the last, immediately preceding fluid), a quantity 
of exceedingly minute granules appear in it, most of which, 
on account of their minuteness, look like mere black points. 
Iodine then seems to colour the fluid a somewhat darker 
yellow. The granules, however, when their size is sufficiently 
large to render their colour perceptible, become of a dark 
browuish-yellow under its influence. 

It is in this mass that organization always takes place, and 
the youngest structures are composed of another distinct, per- 
fectly transparent substance, which presents an homogeneous 
colourless mass when subjected to pressure; when dried it 
imbibes water and swells; it is not at all affected by tincture 
of iodine, nor does it ever imbibe it; after pressure it appears 
as colourless as before, and is so completely transparent as to 
be altogether invisible when not surrounded by coloured or 
opaque bodies. This substance frequently occurs in plants (for 
example, in great quantity, together with a little starch, in 
peculiar large cells in the tubers of Orchis) ; for brevity’s sake 
I shall call it vegetable gelatine; and am inclined to class 
under this head, as mere slight modifications, pectine, the basis 
of gum tragacanth, and many of those substances which are 
usually enumerated under the term vegetable mucus. 

It is this gelatine which is ultimately converted by new 
chemical changes into the actual cellular membrane, or struc- 
tures which consist of it in a thickened state, and into the 
material of vegetable fibre. 

I now pass on to our subject itself. There are two situa- 
tions in the plant in which the formation of new organization 
may be observed most easily and clearly, in consequence of 
there being cavities closed by a simple membrane, viz. in the 
large cell, which subsequently contains the albumen of the 
seed, the embryonal sac, and in the extremity of the pollen- 
tube, from which the embryo itself is developed. The em- 
bryonal sac never contains starch originally, but probably, in 
most instances, the saccharine solution (which gives the sweet 
taste to unripe pod-fruits and the Cerealia), or gum. 

The pollen, on the contrary, always contains starch, or the 
above-mentioned granulous mucus representing it, as an essen- 
tial constituent part. The so-called vegetable spermatozoa 


238 CONTRIBUTIONS TO 


will, probably, on more accurate investigation, be mostly re- 
duced to one of these substances. These substances, however, 
soon become dissolved, and converted either into sugar or gum; 
both changes take place at times, even before the pollen-grain 
has commenced to send forth tubes upon the stigma, frequently 
during the gradual descent of the pollen-tube through the 
style to the ovule; so that in some cases unaltered starch may 
still be found even in the embryonal extremity. 

At both these situations the before-mentioned minute 
mucus-granules are very soon developed in the gum, upon 
which the solution of gum, hitherto homogeneous, becomes 
clouded, or when a larger quantity of granules is present, 
more opaque. Single, larger, more sharply defined granules 
next become apparent in the mass (fig. 2, the upper part) ; 
and very soon afterwards the cytoblasts appear (fig. 2, the 
lower part), looking like granulous coagulations around the 
granules. The cytoblasts, however, grow considerably in this 
free state; and I have observed, in Fritillaria pyrenaica for 
instance, a gradual expansion from 0:00084 to 0-001 Paris inch. 

So soon as the cytoblasts have attained their full size, a 
delicate transparent vesicle rises upon their surface. This is 
the young cell, which at first represents a very flat segment of 
a sphere, the plane side of which is formed by the cytoblast, 
and the convex side by the young cell, which is placed upon 
it somewhat like a watch-glass upon a watch. In its natural 
medium it is distinguished almost by this circumstance alone, 
that the space between its convexity and the cytoblast is per- 
fectly clear and transparent, and probably filled with a watery 
fluid, and is bounded by the surrounding mucus-granules 
which have been aggregated together at its first formation, 
and are pressed back by its expansion, as I have endeavoured 
to represent it in plate XV, figs. 4, 5,6. But if these young 
cells be isolated, the mucus-granules may be almost entirely 
removed by shaking the stage. They cannot, however, be 
observed for any length of time, for in a few minutes they 
become completely dissolved in distilled water, leaving only 
the cytoblasts behind. The vesicle gradually expands and be- 
comes more consistent (fig. 1, 4), and, with the exception of the 
cytoblast, which always forms a portion of it, the wall now con- 
sists of gelatine. The entire cell then increases beyond the 


PHYTOGENESIS. 239 


margin of the cytoblast, and quickly becomes so large that the 
latter at last merely appears as a small body enclosed in one 
of the side walls. At the same time the young cell frequently 
exhibits highly irregular protrusions (fig. 1, c), a proof that 
the expansion by no means proceeds uniformly from one point. 
During the progressive growth of the cell, and evidently arising 
from the pressure of the neighbourmg objects, the form be- 
comes more regular, and then also frequently passes into that 
of the rhomboidal dodecahedron, so beautifully defined @ priori 
by Kieser. (Compare fig. 1, from 4 to e, with fig. 8.) The 
eytoblast is still always found enclosed in the cell-wall, in 
which situation it passes through the entire vital process of the 
cell which it has formed, if it be not, as is the case in cells 
which are destined to higher development, absorbed either in 
its original place, or after having been cast off as a useless 
member, and dissolved in the cavity of the cell. So far as I 
could observe, it is only after its absorption that the formation 
of secondary deposits commences upon the inner surface of 
the cell-wall (fig. 9). 

As a general rule, it is rarely that the cytoblast accom- 
panies the cell which it formed through its entire vital process; 
nevertheless, it is, 

1. Characteristic of the families of the Orchidee and Cactee, 
that in them a portion of their cellular tissue remains in a 
lower stage of development during the entire period of life. 

2. In various plants it occurs that cellular tissue, which has 
merely a transitory signification, is not perfectly developed, 
but retains the cytoblast, and is absorbed together with it at a 
subsequent period. Yet I have also remarked that the latter 
in the middle period of its existence lost much of its distinct- 
ness and sharpness of outline, which, however, reappeared when 
absorption commenced; for example, in the nucleus of the 
ovule of Abies excelsa, Tulipa sylvestris, and Daphne alpina. It 
is most extraordinary that some physiologists should have felt 
prepared to deny the fact, that absorption takes place in plants, 
since even very considerable portions of cellular tissue of the 
nucleus of the ovule, for instance, become completely fluid 
again, and are received into the common mass of the sap. It 
is true this only takes place so long as the cell still consists 
of the simple original membrane, and is not so far advanced 


240 CONTRIBUTIONS TO 


in its individual development that its wall is thickened by 
secondary deposits. 

3. The cytoblasts also remain persistent in the pollen-gra- 
nules in some rare instances ; such is the case in some, perhaps 
in all the Adzetine. The lenticular cytoblast has already been 
observed by Fritsche in Larix europea, but the true nature 
of it was not recognised. 

4, Lastly, many hairs, particularly such as exhibit motions 
of the sap within their cells, retain the cytoblasts (e, f, fig. 25). 
It is at the same time remarkable, and a proof of the close re- 
lation which the cytoblast bears to the whole vital activity of 
the cell, that the little currents which frequently cover the 
entire wall like a network, always proceed from and return to 
it, and that when in statu integro it is never situated without 
the currents (fig. 25). 

I have observed the above-described development of the cells 
throughout its entire course in the albumen of Chamedorea 
schiedeana, Phormium tenax, Fritillaria pyrenaica, Tulipa sylves- 
tris, Elymus arenarius, Secale cereale, Leucoji spec., Abies excelsa, 
Larix europea, Euphorbia pallida, Ricinus leucocarpa, Momordica 
elaterium, and in the embryonal extremity of the pollen-tube 
of Linum pallescens, Cinothera crassipes, and many other plants. 
It was in the summer of 1837, after this treatise had been 
written, that I first began to examine the Leguminose, and 
found to my surprise that these plants, so constantly imvesti- 
gated and everywhere employed as illustrations for the history 
of vegetable development, afforded the most beautiful and ready 
opportunities for the study of this process, which had been 
overlooked by all observers. No one, however, had considered 
the saccharine fluid contained in the embryonal sac as worthy 
of examination. 

Without exactly tracing the entire course of the formation of 
the cells through all its details, I found the cell-nuclei, previous 
to the appearance of the cells, floating loose in the fluid in 
very many plants. Finally, I have not met with a single ex- 
ample of newly-developed cellular tissue, the cambium excepted, 
in which the cytoblasts were wanting. I therefore consider that 
I am justified in assuming the process above described to be 
the universal law for the formation of the vegetable cellular 
tissue in the Phanerogamia. 


PHYTOGENESIS. 241 


My observations are much more limited with respect to the 
Cryptogamia; nevertheless, I found the cytoblasts in the sporidia 
of the Helvelloids, where, however, in consequence of their great 
transparency, they are only perceptible with a very strong 
magnifying power, and after the field has been much darkened. 
I have seen them in the large yellowish cells in the interior of 
the so-called anthers in Chara vulgaris. I also observed their 
development into cells in the sporules of Marchantia poly- 
morpha, oue of which, pushing the original wall of the sporule 
before it, forms the long capillary root (pl. I, figs. 18-20). 

It is evident from the foregoing, that the cytoblast can never 
he free in the interior of the cell, but is always enclosed in the 
cell-wall, and (so far as we can learn from the observation of 
those cytoblasts which are sufficiently large to allow of this 
very difficult investigation) in such a manner that the wall of 
the cell splits into two lamine, one of which passes exterior, 
and the other interior to the cytoblasts. That upon the inner 
side is generally the more delicate, and in most instances only 
gelatinous, and is also absorbed simultaneously with the cyto- 
blast (figs. 8, 16, 21). In making a section, they are some- 
times detached and scattered over the object, which might lead 
to the supposition that they lay free. It is probable also that 
subsequently, when absorption commences, they do become dis- 
engaged from their connexion with the cell-wall, and a slight 
touch may then be sufficient to move them from this position. 
The cell-wall is often considerably thickened in their neighbour- 
hood, especially when they are somewhat globular; for instance, 
in the pollen-tube, which has become cellular in certain Orchidee 
(figs. 16, 20). 

Meyen, who should always be consulted with reference to 
anatomical questions, has endeavoured, in his Physiologie, vol. i, 
p. 45, &ec., to establish the opinion, that the cell is formed 
of spiral fibres which lie closely one upon another, founding 
his view in a most ingenious manner upon his own beautiful 
observations on the relations of structure in fully-developed 
cells. My direct observation, which may easily be repeated 
by every one, shows, it is true, quite a different mode of forma- 
tion; I must, however, bring the facts related by Meyen into 
unison with my discovery, in order not to permit an apparent 
contradiction to remain unresolved. 

16 


242 CONTRIBUTIONS TO 


Meyen himself correctly observes, when treating of those 
spiral tubes whose very narrow fibres lie close upon one 
another, that an enveloping membrane could not indeed be 
observed, but that this by no means justified our concluding 
on its absence. For if the thickenings of the cell-walls which 
are formed in most, perhaps in all, cases in spiral lines, in 
those instances in which they make their appearance early, 
whilst the original cell-wall itself is yet in statu nascentie and 
soft, become firmly connected with the latter; and if at the 
same time the separate coils of the spiral fibre le perfectly 
close one upon another, so that with our present microscopes 
no space remains perceptible between them,—it naturally fol- 
lows that on tearing the entire membrane (the so-called un- 
rolling of the spiral vessels), the fracture in the direction of 
the coils of the fibre must be so sharp that our instruments 
could not possibly show the inequalities. At the same time 
it should be remembered that the original cell-membrane, 
especially in long hair-cells, frequently undergoes so great an 
expansion that it must at last become infinitely delicate, so 
that even the thinnest and apparently most simple cell-wall 
does not exclude the possibility of its bemg composed of the 
original membrane and the secondary deposit. If, then, we 
proceed from those spiral cells and vessels whose coils are so 
far distant from one another as to admit of no doubt with respect 
to the existence of an external enveloping membrane, and if 
we trace the presence of this membrane through all the forms 
of the constantly approximating coils of the fibre, until only 
the feebleness of our optical instrument renders further direct 
observation impossible, the laws of sound analogy require that 
we should, in such instances, also admit the presence of a 
similar membrane. There is yet a more direct mode of proof, 
namely, the investigation of the history of the development. 

It is an altogether absolute law, that every cell (setting 
aside the cambium for the present) must make its first appear- 
ance in the form of a very minute vesicle, and gradually 
expand to the size which it presents in the fully-developed 
condition ; an extended investigation of this formative process 
also invariably shows that a cell never exhibits a trace of 
spiral formation, discoverable either from its aspect, or 
on tearing it, previous to its complete development, i.e. before 


PHYTOGENESIS. 243 


it has absorbed the cytoblast. _ In all spiral cells, particularly 
such as exhibit detached fibres, we find the walls of the fully- 
developed cells to be perfectly simple at the commencement. 
For instance, I remarked this in the outer parchment-like layer 
of all aérial roots." Meyen discovered the spiral fibres in 
Oncidium altissimum, Acropera Loddigesii, Brassavola cordata, 
Cyrtopodium speciosum, Aérides odorata, Epidendron elongatum, 
Cattleya Forbesii, Colax Harrisonii, and Pothos crassinervia. 
This is still more evident in the true cortical layer of those 
aérial roots, where I discovered in Colax, Cyrtopodium, and 
Acropera the far more beautifully developed and much broader 
spiral fibres. There is no trace of them to be found in quite 
young aérial roots, and their formation pertains decidedly to 
a process of lignification. 

We find further evidence that the spiral fibres do not occur 
until a subsequent period in the pericarp of the Casuarina, 
the cells of which, previous to or shortly after impregnation, 
do not evince a trace of spiral formation. Meyen, in his 
Physiologie, has taken too little notice of these fibre-cells in 
the envelopes of many seeds, which is the more to be re- 
eretted, as these interesting and sometimes extremely pretty 
formations promise some explanation respecting the physiology 
of the cell-life, especially if the opportunity should occur of 
investigating the individual development of several of them 
accurately. I may be permitted to communicate a few obser- 
vations on this subject. 

Their occurrence is more extensive than is generally sup- 
posed. They are found in the hairs of the pericarp in some 
Composite, where they were found by Lessing in Perdiciwm 
taraxaci and Senecio flaccidus, and by myself in Trichocline 
humilis and heterophylla. 


! Meyen, in his Phytotomie, p. 163, called this an outer cortical layer, which was 
situated on the true epidermis of the aérial roots. Some doubts have recently been 
raised as to the correctness of this view. It may, however, be almost incontestably 
proved, since the cellular layer, which Meyen calls epidermis, possesses actual sto- 
mata, which, in consequence of their being covered, usually indeed occur only in a 
rudimental form, frequently exhibit a more complicated structure, although deviating 
only in appearance, as in Aérides odorata, but often likewise appear of quite the 
ordinary form, as in Pothos crassinervia. Moreover it was not Dutrochet, as would 
seem from Meyen’s Physiologie, p. 48, but Link, who first drew attention to this 
layer. 


244 CONTRIBUTIONS TO 


They occur in the epidermis of the pericarp in many Ladiate, 
as in Ziziphora, Ocymum; in most Salvie, for instance, lim- 
bata, hispanica, Spielmanni, &c.; and lastly, in Horminum 
pyrenaicum. My uncle Horkel was familiar with them in all 
these many years ago; Baxter noticed and published their 
occurrence in Salvia verbenacea only. I can add to these 
Dracocephalum moldavica. 

R. Brown discovered them in the parenchyma of the peri- 
carp in the Casuarine, and I in the spongy inflated cellular 
tissue in Picridium vulgare, where they mostly occur in a 
reticular form, and present an extremely beautiful appear- 
ance. 

Horkel also discovered them in the epidermis of the seed 
itself in the Polemoniacee long before Lindley made known 
their presence in Collomia linearis. They occur in Collomia, 
Gilia, Ipomopsis, Polemonium, Cantua, Caldasia, and perhaps in 
the entire family, with the exception of Phlox, with which 
genus Leptosiphon, m which are the first indications of them, 
is closely alhed. Horkel had also studied them in the seeds 
of Hydrocharis, where they occur in the highest degree of deve- 
lopment, long before Nees von Esenbeck published the fact. 
Robert Brown mentions them in the Orchidee, which statement 
I find confirmed as to most of our native species of Orchis. 
I have also discovered very beautiful spiral fibre-cells in the 
epidermis of the seed of Momordica elaterium, and a very deli- 
cate reticular formation of fibres in Linaria vulgaris, Datura 
stramonium, in Salvie, and in several other Labiate ; probably 
it is common to the whole family. 

Lastly, they occur, according to Horkel’s discovery, in the 
parenchyma of the integuments of the seed in Cassyta and 
Punica. 

Whether these formations be studied in their individual 
development in a single species, or in their progressive stages 
in a series of allied plants, some highly interesting general re- 
sults will be obtained in either case. The universal and alto- 
gether absolute fact at which we first arrive is, that the fibres 
are never formed free, but are developed in the interior of 
cells; and that the walls of these cells in the young state are 
simple, and generally very delicate. Corda’s statement re- 
specting spiral cells without an enveloping membrane (Ueber 


PHYTOGENESIS. 245 


Spiral faserzellen, &c., pp. 7, 8) is based upon inaccurate ob- 
servation. 

These cells are at first generally filled with starch; rarely 
with mucus or gum. ‘The starch always passes into the latter 
substance in the progress of development; and this is con- 
verted into jelly, the change, as it would seem, taking place 
from without inwards. This jelly finally is converted at its 
outer surface into vegetable fibre, following the direction of a 
spiral lime, the coils of which are sometimes narrower, some- 
times wider. When these forms are observed in their different 
stages of development and in their various conditions, the idea 
involuntarily forces itself upon the mind that the spiral forma- 
tion is the result of a spiral movement of a fluid on the walls 
of cells between them and the central jelly. Horkel once 
actually observed the motion of small globules between the 
coils of the fibre in progress of formation in Hydrocharis. 

The great variety in the appearance of the fibres seems to 
depend upon the period of their origin, and on modification in 
the chemical changes of the formative material. It probably 
depends solely upon the former circumstance whether the spiral 
fibre lies free in the cell, when it is formed very late, or 
whether it is blended with the membrane of the cell, if its 
development commence at a period when the cell-membrane > 
itself is yet very soft and gelatious, and may consequently 
become agglutinated to the fibre, which is likewise still in a 
gelatinous state.’ This is the case in Casuarina, Cassytha, 
Hydrocharis, Trichocline, Orchis, &c.; in most cases, however, 
the cell-wall is too far developed to unite with the fibre, 
and the latter then les loose in the interior of the cell. In 
rarer instances the material is almost entirely applied to the 
formation of the fibre (always indeed when the fibre coalesces 
with the wall), for example, in Salvia Spielmanni, Mo- 
mordica elaterium. I have reason to suppose that this com- 
plete consumption almost always takes place in spiral vessels, 
and is the cause of their subsequently conveying only air. 
More frequently, however, one or more fibres are formed ; but 
then a great portion of the jelly has still remained uncon- 

' Subsequent researches have produced important modifications in this opinion. 


Consult my essay on the Spiral Formations in Vegetable Cells. Flora, 1839, Nos 
P22 slave 


246 CONTRIBUTIONS TO 


sumed, which, when the cell is moistened with water, comes 
forth in form of an intestine (wie ein Darm hervortritt), and in 
swelling exparids itself over the fibres, thus appearing to sur- 
round them ; this is the case in most Salvie and Polemoniacee, 
in Senecio flaccidus, Ocymum polystachyum and polycladum 
(Lumnitzera, Jacq.) There is an intermediate form between 
this and the former, when the jelly itself forms a broad 
spirally-wound band, which appear upon its surface to be com- 
posed of innumerable delicate fibres; their occurrence in this 
state is very beautifully shown in Perdicium Tarazxaci and 
Ziziphora. A still less advanced stage of development exhibits 
merely a cylinder or cone of gelatine in the interior of the 
cell, the surface of which, however, is marked with delicate 
spiral lines. This is seen in some Salvia, in 8. verticillata for 
example, and in Leptosiphon androsaceum. Finally, the lowest 
stage of development is where the gelatinous cylinder, which 
is furnished with spiral striz, has a cavity in its imterior con- 
taining starch, which has not as yet undergone decomposi- 
tion; this instructive phenomenon is found in Dracocephalum 
moldavica, Ocymum basilicum, and some allied species. In illus- 
tration of the above, consult plate 2, figs. 1-10, with their 
explanations. 

Before quitting the subject of spiral fibre, I will merely 
add, what indeed has been of late admitted by every good 
observer, that the only difference between spiral cell and spiral 
vessel consists in the dimensions, although constant transitions 
may be observed between them just as well as between the cells 
of the liber and the parenchyma; and consequently, as regards 
this doctrine at least, there is no longer any place for natural- 
philosophical phantasies about the arrestment of ideal forms of 
higher types, and such like empty words. That which forms a 
liber-cell out of a round cell, the preponderating expansion of an 
organ lengthwise, is also that which transforms the spiral cells 
(the vermiform bodies) into spiral vessels. The function of the 
spiral fibre, however, is, as every candid vegetable physiologist 
will certainly admit, entirely unknown to us at the present 
time. It is certain that spiral vessels and spiral cells occur in 
the living plant quite as frequently filled with sap (in the 
younger vegetating portions) as with air (in the older organs 
which have attained their full size); and it is this which has 


PHYTOGENESINS. 247 


given rise to the conflicting views of authors. But the same 
also occurs in all cells under certain circumstances, and the 
influence of the spiral fibre remains meanwhile altogether ob- 
scure and unexplained. Perhaps the foregoing may render it 
probable that the spiral is everywhere only a secondary varia- 
tion of form in the product of the vital power (the fibrin) pro- 
duced by a different tendency of the vital activity of the cell, 
so soon as this is compelled, as a certain stage of its develop- 
ment, to give up its independent individuality, and enter as an 
integral portion into the complex of the entire plant. 

I also think that we may venture, in conclusion, to deduce 
from the data above enumerated, that this indication of a spiral 
formation is the surest sign that we have no longer anything 
to do with the simple cell-membrane. 

I now return, after this somewhat lengthy digression, to my 
subject. The process of cell-formation, which I have just 
endeavoured to describe in detail, is that which I have observed 
in most of the plants which I have investigated. There are, 
however, some modifications of this process which make the 
observation of many parts very difficult, and sometimes indeed 
render it impossible, although, notwithstanding this, the law 
remains undisturbed and universally valid, because analogy 
requires it, and we can fully explain the causes of the impossi- — 
bility of direct observation. 

The difficulties which I now notice depend especially upon 
the physical and chemical properties of the substance which 
precedes the formation of cells. The materials enumerated 
above are to be regarded as scarcely anything more than sepa- 
rate facts, which, for the purpose of giving a general view and 
rendering the classification more easy, I have intentionally 
selected from the organic chemical processes of vegetable life, 
which are constantly in operation, and with which we are as 
yet totally unacquainted. Almost all these materials con- 
stantly exist together in the living plant, and it is merely 
their preponderance in a greater or lesser degree which enables 
us to say that the cell contains amylum or gum, and so forth. 
Only towards the termination of the individual life of the 
cells do we find them filled with a less number of different 
substances ; the cells which contain ethereal oil are probably 
the only instances in which we find but a single one. 


248 CONTRIBUTIONS TO 


If we now assume a cell to be completely filled with a 
transparent solution of sugar in which there is rapidly gene- 
rated just so much gum, as may form, by an equally quick 
conversion into jelly, a delicate cell-membrane, the exist- 
ence of which we cannot possibly recognise with the micro- 
scope, in consequence of the similar refracting power of the 
wall, the contents, and the surrounding medium ; it then be- 
comes exceedingly probable that a number of such formative 
processes may go on which escape our observation, and become 
known to us only in their results, when, after the absorption 
of the parent-cell, we suddenly find two new ones in its place. 
If, on the other hand, our attention has been previously directed 
to this process, we have, in the application of reagents, espe- 
cially iodine, which is quite indispensable to the physiological 
botanist, several means of rendering it visible in instances 
where it is suspected to be going forward. Gradual transition 
to the completely invisible processes are readily found by more 
extended investigation ; I will just mention one of the most 
difficult instances which I have met with, by way of example. 
It occurs in the germination of the sporules of Marchantia poly- 
morpha. Only a few, generally only from two to four of the 
cell-nuclei which appear in the sporules, serve for the formation 
of cells; the others become quickly enveloped with chlorophyll, 
and are thus withdrawn from the vital process. The transparent 
fluid, however, in which these cytoblasts float, passes through 
the remaining stages of the metamorphosis into cell-membrane 
only just at the boundary of the latter, and with such rapidity 
that the exceedingly delicate young cells cannot be distin- 
guished by anything else than a minute, generally more or less 
uninterrupted circle of infinitely small, black granules, and by 
a scarcely perceptible greater transparency of the contents of 
the newly-formed cells in comparison with that of the parent- 
cell, and finally, under the most favorable circumstances, by the 
spot at which the newly-developed cells come into contact, the 
point of juncture being still covered by the membrane of the 
parent-cell. (Pl. I, figs. 18-20.) This may perhaps be general 
in the Cryptogamia, and especially in water plants, and probably 
Mohl’s division of the cells of Conferve may be thus explained. 

If we consider, however, that there are undoubtedly many 
plants, among which the Fungi and infusorial A/ge should pro- 


PHYTOGENESIS. 249 


bably be classed more especially, in which we are, as yet at 
least, totally unacquainted with the cytoblasts, in consequence 
of their absolute minuteness and transparency ; if we further 
bear in mind that the nucleolus in the cell-germ, even 
im the larger cytoblasts, frequently appears immeasurably 
small, or even entirely escapes the eye with the highest mag- 
nifying power; and, lastly, if we deduce from what has been 
previously stated, that nevertheless this granule, which can no 
longer be rendered perceptible, probably furnishes in the suit- 
able medium a sufficing cause for the formation of a cytoblast 
which serves as an introduction to the whole formative process 
of the cells; then, indeed, we are forced to confess that the 
imagination obtains ample latitude for the explanation in every 
case of the generation of infusorial vegetable structure, even 
without the aid of a deus ew machina (the generatio spontanea). 
But my present object is to communicate only facts and their 
immediate consequences, and not to dream; I will therefore 
rather add a few more observations on the growth of the 
plant. 

What is meant by to grow ? isa question to which every child 
quickly replies, “ when I am getting as big as father.” There 
is truth in this answer, but not sufficient to satisfy science. 
Words have no value in themselves, but are like coin, merely — 
tokens of a value not exhibited in specie, in order to facilitate 
commerce. And to carry the simile further, insecurity in this 
intellectual property, and frequently bankruptcy results, if 
this coimage has not its unchangeable, accurately-determined 
standard ; in a word, the utility of a scientific expression de- 
pends upon the accurate definition of the idea on which it is 
based. Unfortunately the perplexity of our social relations has 
caused us to forget entirely the original meaning of money, 
the sign has become to us the thing itself; may some good 
genius protect us from similar mistakes in our intellectual life. 
We must here be on our guard against two dangerous rocks: 
first, when we transfer words from one science to another, 
without first accurately testing whether they fit their new 
situation as respects all their accompanying significations also ; 
and, secondly, when we voluntarily lose sight of the significa- 
tion of a word consecrated by the spirit of the language and 
its historical development, and employ it without further cere- 


250 CONTRIBUTIONS TO 


mony in compound words, where perhaps, at the most, only 
some unessential part of its signification suits. 

Thus E. Meyer, for example (Linnea, vol. vii, p. 454), after 
repeating the well-known experiments of Duhamel, lays down 
this position: “the law of the longitudinal growth of the 
internodes is to grow in a direction from above downwards.” 
He requires this position for his theory, and must consequently 
defend it in every way, although he himself confesses that this 
reversed growth must appear paradoxical to every one of his 
readers. He would never have arrived at this position if he 
had more accurately analysed the word “grow” (with which 
animal physiology had rendered him familiar), with reference 
to its applicability to the plant; he would soon have discovered 
that the generation of new cells, and so far the actual growth 
of the plant, constantly takes place in its outermost portions 
in an upward direction, and that his very simile of the building 
up a voltaic pile is exceedingly well adapted to refute himself. 
The experiments of Duhamel and Meyer would have no fur- 
ther result than to show that the inferior, that is, the earliest 
generated, older cells of the internode possess a greater capa- 
bility to extend in the longitudinal direction, and retain this 
power longer than the younger cells. 

We have an excellent illustration of the second point in the 
proposition so frequently expressed of late, that the stem of 
the plant is composed of the coalesced petioles. The word 
“coalesce” (verwachsen, to grow together) has possessed, how- 
ever, from time immemorial, both in ordinary life and in 
science, the signification that two or more originally and 
naturally separate parts have become by the process of growth 
either abnormally or, under certain circumstances, normally 
united. If therefore the word “coalesce” be applied to the 
stem of the plant, an organ, which, in every period of its ex- 
istence, under all forms of its appearance, is a simple and 
undivided one, and at the origm of the plant even constantly 
appears earlier than the leaves with their petioles, it certainly 
involves a mischievous abuse of language, by which science 
itself can gain nothing, and will even lose in the estimation 
of the intelligent non-professional man, who sees through such 
a play upon words. What would the zoologist say were we to 
regard the trunk as a coalescence of the extremities. 


PHYTOGENESIS. 251 


I return then to my question: what is the meaning of to 
grow? In hackneyed phrase we are told, “ To grow signifies 
merease of the mass of an individual, and takes place in the 
inorganic world by juxtaposition, in the organic by intus- 
susception.” Have we gained anything for vegetable physi- 
ology by this reply? Ithink not. If the plant is to grow 
by intussusception, then I say it consists of an aggregate of 
single, independent, organic molecules, the cells; it increases 
its mass by new cells being deposited upon those already ex- 
isting ; consequently by juxtaposition. But the single cell in 
the progress of its expansion, which frequently reaches an 
enormous bulk in comparison with its original size (I will 
merely remind the reader of the pollen-tubes), also increases 
in substance in the interior of its membrane, and by this 
means also the mass of the entire plant is increased ; it con- 
sequently grows by intussusception also. Finally, after a certain 
period the cell deposits new organic material in layers upon its 
primitive membrane ; thus another form of juxtaposition, which 
still, however, belongs to the cycle of vegetable vitality. It 
hence becomes readily apparent that, in respect to scientific 
botany, the idea “grow” still requires a new foundation in 
order to be capable of being applied with certainty. 

Of the three instances just cited, the second and third 
belong more to the individual life of the cells, and are of 
secondary importance only, as respects the idea of the whole 
plant, regarded as an organism composed of a certain number of 
cells. The plant considered in its totality increases its mass, that 
is, the number of the cells composing it, in the first way only. 

We must therefore here discriminate three processes essen- 
tially distinct from each other in a physiological sense, which, 
when strictly regarded, scarcely find an analogy in the other 
kingdoms of nature. 

1. The plant grows, that is, it produces the number of cells 
allotted to it. 

2. The plant unfolds itself by the expansion and develop- 
ment of the cells already formed. It is this phenomenon 
especially, one altogether peculiar to plants, which, because it 
depends upon the fact of their being composed of cells, can 
never occur in any, not even the most remote form in crystals 
or animals. 


252 CONTRIBUTIONS TO 


3. The walls of the fully-developed cells become thickened 
by the deposition of new matter in layers, a process which, in 
accordance with the old rule, a potiori fit denominatio, may be 
most aptly termed the lignification of the plant. 

If, in respect to the growth of the plant, we now hold to 
the literal sense conveyed under No. 1, then this question 
must arise,—Where are the new cells formed? Here three 
instances comprise all possible replies. Namely, the new ceils 
are either formed outside on the surface of the entire previous 
mass, or in its interior; and in that case again either in the 
intercellular spaces or in the cells themselves; guartum non 
datur. 

Mirbel, in two extremely ingenious and profound memoirs 
on the Marchantia polymorpha, which he presented to the 
French Academy in 1831 and 1832 (p. 32), has expressed the 
opinion, that all the three cases just now mentioned as possible 
do actually occur in plants. Without intending here to anti- 
cipate what follows, I must remark that only one case (the 
formation of new cells within the old ones) appears to be 
proved by his direct observations. The second case is merely 
a conclusion drawn, and the germination of the sporules of the 
Marchantie, which was to elucidate the third case, has been 
observed by me to be quite different, as I have already repre- 
sented above. 

Finally, however, we have yet to examine whether the differ- 
ence of the organs may not establish such a physiological 
difference of growth as may merit our attention. We may 
distinguish here four instances. We observe: 1. The develop- 
ment of the plants in the upward direction (im puncto vege- 
tationis, C. Fr. Wolff). 2. The elongation downwards. We 
thus comprise the formation of the necessary orgaus of the 
plant, the stem, the leaves (with their metamorphoses), and the 
root. 8. We have to keep in view the production of accidental 
organs, for example, bulbs, &c. And, 4. We find an annual 
thickening of the axile formations, the development of the 
woody stem. 

Let us now see which of the three possible modes of forma- 
tion of new cells is actually realised in each of the cases just 
enumerated. I have already explained how the new cells are 
developed in the embryonal sac ; in other words, within a large 


PHYTOGENESIS. 253 


cell. A similar process occurs in the embryonal end of the 
pollen-tube, consequently in a highly elongated cell; I shall 
now proceed to describe the further development of the embryo. 
After the first cells, generally few in number, are formed, they 
rapidly expand to such an extent that they fill the pollen-tube, 
which soon ceases to be perceptible as the original enveloping 
membrane ; but at the same time several cytoblasts originate 
im the interior of each of these cells, and generate new cells, 
on the rapid expansion of which the parent-cells also cease to 
be visible and become absorbed. The same process is repeated 
indefinitely. But since the newly-generated cells have con- 
tinually less room to expand, and therefore constantly become 
smaller, the previous transparency is soon lost in consequence 
of the continual production of new cytoblasts in the interior, 
and the tissue becoming more and more compressed ; and from 
this stage to the perfect completion of the embryo we are con- 
ducted by the clearly logical inference that the process thus 
introduced continues the same, since no new force comes into 
operation which could induce us to assume a sudden variation 
of the vital action, more especially as we soon meet with the 
same manifestation of the vegetative power again. 

Meanwhile the seed germinates, and the embryo becomes a 
plant; and then indeed the question may arise,—Does the pro- 
cess of life continue the same thenceforward in the internodes 
and foliaceous organs? Now we are here very quickly con- 
vinced of the negative, that is, that a formation of new cells on 
the surface of the existing organs does not take place. The 
surface is always smooth, and generally covered in a very early 
state with a kind of epidermis, the outer layer being more 
transparent and almost as clear as water; and we never find 
even an indication of a newly-formed cell upon the surface. 

But if the embryo be the type of the entire plant, and the 
latter do not present anything which is not a repetition of its 
organs, if we have found the growth of the embryo to consist 
only in the formation of cells within cells, we may then expect 
to find the same result also in the process of the growth of the 
whole plant. It is especially a foliaceous organ, the anther, 
which has hitherto been studied and followed in its develop- 
ment by many celebrated men (particularly well by Mirbel) ; 
and here it is quite decided that the increase of cells takes 


254 CONTRIBUTIONS TO 


place within the old ones. It is also certain that in this case 
the formative process accords with that above described. R. 
Brown and Meyen have enumerated many instances where 
they observed the cytoblast in very young pollen-cells. In 
Pinus, Abies, Podostemon, Lupinus and others, I have traced 
the development of the pollen after Mirbel perfectly; I have 
distinctly observed the cell-nuclei and their development into 
new cells within one another in Adzes, never having missed the 
cytoblast in young cells. 

Now if the pollen-grains be nothing more than converted 
leaf-parenchyma, if the anther be merely a metamorphosis of 
the leaf, we may certainly infer inversely that the process 
which we have observed in it, and which characterized the 
formation of the embryo and cotyledons (as prototypes of the 
leaf) will be again found in all foliaceous organs. For the 
same reason which was stated with respect to the later stages 
of the development of the embryo, actual observation is infi- 
nitely difficult in this case. I have nevertheless examined a 
great many buds in reference to this point, and have most 
decidedly convinced myself of the identity of the process both 
in the constantly elongating apex of the axis, and in the leaves 
which always originate somewhat beneath it. Succulent plants, 
the Aloinee and Crassulacee, are best adapted for this purpose. 
Crassula Portulaca seemed to me most advantageous, for in it 
I first succeeded in separating some cells from their connexion, 
in whose interior young cells were already developed, without, 
however, entirely filling the parent-cell. But having once be- 
come familiar with the subject, I was afterwards able to detect 
these individualities from amongst the apparently semi-organised 
chaos in all other plants. Another circumstance indeed pre- 
sents itself here, which renders the subject much more difficult 
than in the case of the embryo. For, independently of the 
minuteness of the cells, their walls, in those parts of the plant 
which are just newly formed, still consist merely of jelly, and 
are so delicate that it is exceedingly difficult to separate the 
parts intended for examination without completely destroying 
the organization. (Compare plate I, figs. 22-4.) 

This process is more easily perceptible in articulated hairs, 
and in such as have a head consisting of several cells, where 
the same appearances which I have so frequently observed in 


PiTYTOGENESIS. 255 


the young embryo, and such as Mirbel has so_ beautifully 
described in the development of the gemmz in the cups of 
Marchantia, may be readily and beautifully seen; for example, 
in the common potato. Meyen has also made similar ob- 
servations, although he still expresses himself with some doubt 
on the subject. (Wiegmann’s Archiv, 1837, vol. ii, p. 22.) 

It is not until after as many cells are formed as the organ 
requires for its completion that the cell-walls become firmer, 
and then commences the unfolding of the organ by the mere 
expansion of the cells already formed. 

But I must here enter somewhat more into detail, in order 
to explain the probable origin of the vascular bundles and 
epidermis. At a somewhat early period a stripe of more trans- 
parent cells is defined in the axis of the leaf which is in the 
act of formation, within which no more new ones are deve- 
loped, and these cells soon considerably exceed in size those of 
the remaining mass, which are constantly becoming smaller 
by continual division. These cells are the basis of the future 
vascular bundle which forms the midrib of the leaf; for whilst 
the parenchymatous cells subsequently expand in every direc- 
tion, these are developed in their longitudinal dimension only, 
and are thus enabled, although fewer in number, to follow the 
expansion of the other cells in the longitudinal direction of the 
leaf. It is not till a later period that these cells, in conse- 
quence of a difference in the depositions in their interior, be- 
come distinguished into spiral vessels and cells of the liber. 
The spiral vessels are always first perceptible in the newly- 
formed parts, and in the entire bud also, in the immediate 
neighbourhood of the old, previously-formed spiral vessels; and 
they proceed in this manner downwards from the stem into 
the new parts. I do not understand therefore what is intended 
when the fibres of the stem are regarded as descending from 
the buds; one might just as well conceive the river to run 
from the ocean to its source. 

A similar process occurs in the development of the side 
nerves of leaves. The formation of new cells generally ceases 
at an early period in the outermost layers of cells. The cells 
there are soon filled with a limpid fluid, and, by the expansion 
of the subjacent parenchyma, naturally become superficial, flat, 
and expanded. 


256 CONTRIBUTIONS TO 


The cells of the vascular bundle and of the epidermis 
appear in this way to be less potentialized,—are as it were 
cells of lower dignity than those of the parenchyma; and 
perhaps this physiological peculiarity is connected with the 
fact, that they more rarely secrete peculiar chemical substances, 
but for the most part become thickened only by depositions 
within their walls of new vegetable fibrous (or more correctly 
membranous) substance. I cannot forbear venturmg some 
suggestions in this place, which are perhaps less closely con- 
nected with the subject of this memoir, but which may possibly 
at some future time be of importance for the understanding of 
the entire plant. Let us recapitulate the process of growth of 
the plant just now represented. A simple cell, the pollen-tube, 
is its first foundation. Within this, cells are generated; in them 
new cells are developed, and so forth, throughout the entire 
life. But here the above-mentioned mode of the origin of the 
vascular bundles and of the epidermis in relation to the paren- 
chyma would indicate, that the lower the dignity of the cell, 
the greater power does it possess, in the first place, of expand- 
ing and extending in length, and the less capacity does it 
possess, in the second place, of forming peculiar finer sub- 
stances in its interior. If now the potentialization (poten- 
zirung) of the cells proceed throughout the entire growth of 
the plant, there thence results a constantly advancing approxi- 
mation of organs otherwise kept asunder, and continually rising 
ennoblement of the substances developed in the cells. Conse- 
quently, the lower parts of the internodes will appear to be 
more elongated than the upper; the leaves and young shoots 
(summitates herbarum, Pharmacol.) to contain nobler saps than 
the stem; the members become shortened as they approach 
nearer to the upper terminal point of the plant, the leaves 
come closer together, and the result of the continually 
increasing potentialization of the cell, of the constantly dimi- 
nishing expansion in length, of the constantly advancing ap- 
proximation of the lateral organs, of the constantly rising 
ennoblement of the substances developed, is, finally, the flower 
in its individualised distinctness, with its splendour of colour, 
its perfume, and its mysterious capacity of determining, by 
means of its juices, a single cell to be developed afresh into an 
independent plant, and to pass anew though the same cycle. 


PHYTOGENESIS. 257 


I return, after this digression, to my subject. So far I be- 
lieve I have demonstrated tolerably conclusively, and in accord- 
ance with nature, that the entire growth of the plant! consists 
only of a formation of cells within cells. Let us now pass on 
to the root. I can contribute but very little to the explana- 
tion of this part of the subject; for I have not as yet succeeded 
in arriving at any satisfactory result, from the somewhat limited 
researches which I have instituted ; for instance, I have been 
altogether unsuccessful in deciding the question as to whether 
a fluid is secreted at the extremity of the radicle, in which 
new cells are developed. On the other hand, it is certain that 
there exists in the extremity of the root a concavo-convex 
mass (a meniscus) of cellular tissue, in which the process of 
cell-formation takes place in the same manner as in the parts 
of the plants which grow in the ascending direction. <A chief 
cause of the elongation of the root consequently consists m 
this,—that new cells are continually formed in the interior of 
the existing cells, on the convex side of that mass of cells, 
while on the concave side, the cells already formed expand 
simultaneously, and chiefly indeed in the longitudinal direction, 
and in this way constantly push the extremity of the root 
before them. 

The third case, the formation of the accidental organs of the 
plant, I must entirely pass over, as I am altogether unpro- 
vided with any personal observations upon the subject. Pro- 
bably, however, the process here is the same as in the previous 
cases, for Meyen (Physiologie, vol. i, p. 209) observed the cell- 
nuclei in germinating tubers of Orchidee. Analogy also leads to 
a similar conclusion, since all these parts are nothing more than 
morphological modifications of organs which have been already 
treated of in this memoir. ‘The fourth pomt, however, still 
remains for discussion, namely, the increase in thickness of 
plants which form woody stems (Dicotyledons). The origin and 
signification of cambium is the nut on which so many young 
phytologists have already broken their milk-teeth, the Gordian 
knot which so many botanical Alexanders have cut instead of 
untying, and the enigma, for the solution of which almost all 
the Coryphei of our science have laboured with more or less 

' I beg to observe, that generally throughout the entire memoir phenogamous 
plants only are referred to. 


17 


258 CONTRIBUTIONS TO 


success. My researches also with respect to this newly-arising 
formative layer between bark and wood are by no means 
concluded. 

Before, however, I proceed to communicate my observations 
on this subject, it is necessary once more to take up the ques- 
tion of the individuality of plants. I have already remarked 
above that, in the strictest sense of the word, only the separate 
cell deserves to be called an individual. If we go a step 
further, we might define each axis with its lateral organs to be 
individual beings. If, however, we disregard this circumstance 
of the plant bemg composed of cells and similar axes, and con- 
ceive the term individual, as applied to the organic world, to 
signify a body which cannot be divided into two or more simi- 
lar ones without the abolition of its idea of totality, and whose 
vital process has a fixed point of commencement and termi- 
nation in definite periodicity, it thence follows that the her- 
baceous (planta annua) and the true biennial plants, which 
flower in the second year, and then die off entirely, are the only 
ones which can be regarded as individuals in the vegetable 
kingdom. The idea of individual life also necessarily requires 
as a characteristic that individual death should be a condition 
of the organization itself. But where such a death does not 
take place as a final termination from internal necessity, as an 
internal preconditioned cessation of the organizing force, there 
also must individuality be out of the question. This is the 
case, however, only in the above-mentioned plants, and from 
them solely, therefore, as from the prototype, must we set out, 
in all researches into the nature and life of the vegetable 
organism. 

In order to facilitate the transition to what is to follow, I will 
now proceed to the exposition of the two different modes of 
propagation. It either takes place by a process which has 
hitherto been called impregnation in plants, and to which a 
sexual difference has been ascribed (Wiegmann’s Archiv, 1837, 
vol. i, p. 290, &c.), or by division ; the plant, for instance, deve- 
loping on itself a perfectly similar imdividual, and then at an 
appointed time dismissing it. This latter, the formation of so- 
called bulbilli, &c. occurs, together with the former, in only a 
small number of plants. We must, however, make ourselves 
somewhat more intimately acquainted with it. This formation, 


PHYTOGENESIS. 259 


for instance, does not always take place in such a manner that 
the parent plant separates itself entirely from them, and scat- 
ters them about singly, but it most frequently forms, previous 
to its own individual death, a peculiar organ, which places the 
offspring in a peculiar vital connexion with one another, and at 
the same time serves as a reservoir for a certain quantity of 
nutritive material, by which the first development of these 
young individuals is facilitated. But in most cases this organ 
is merely a metamorphosis of some other one with which we 
are already familiar, for example, the stem or the root, or, as 
in the potato, the axillary buds; and no scientific person. has 
therefore ever hesitated to speak of these things as mere por- 
tions of a plant, which continue to live as connecting members 
between the younger individuals after the death of that one 
which has generated them. On the other hand, a different 
course has been taken, where stem and root simultaneously, and 
therefore almost the entire totality of the plant, take part in 
the formation; and although the result in this case may per- 
haps be that there can be no question at all of an hetero- 
morphy of a known portion of a plant, still the physiological 
identity in the signification of this and the former part has 
not been maintained with precision, and the view has thus 
been obscured. 

Most manuals are silent upon this subject, as though it 
were quite self-evident that the tree was to be regarded as 
the perfect plant; and I believe it not difficult to prove that, 
where vegetable physiology still lies very deep in error, this par- 
ticular misconception is solely in fault. Two entirely distinct 
ideas have here been confounded, namely, the highest stage of 
development to which vegetable hfe can raise itself, and the 
type upon which the idea of the individual must be based. If, 
then, the first of these ideas may be maintained with regard to 
the tree, still the application of the second to it fails com- 
pletely in every respect, as has been very correctly asserted 
before by E. Meyer (Linnea, vii, p. 424). It necessarily per- 
tains to the notion of a plant, that it produces foliaceous organs 
on its stem, yet there is no tree which has leaves. Paradoxical 
as this may sound, it is still not the less true. It is a fact, of 
which certainly no botanist is ignorant, that no lignified part 
of a plant, even though it be only im its second year, is capable 


260 CONTRIBUTIONS TO 


of producing a leaf; but the direct consequence is by no means 
so generally acknowledged, that for that very reason the woody 
stem cannot come under the idea of plant. Much confusion 
has arisen in our physiology from the error of regarding the 
tree as a single plant, the ideal definition of root, stem, bud, 
&c. have become very vague, and bitter controversies have 
been carried on with respect to the functions of these parts, 
which could have no result, because the one party spoke of 
this, the other of that, this one of the stalk, the other of the 
stem, this of root-fibrils, that of ligneous root-substance. 

The so-called lignified root is, however, just as little a root, 
as the lignified stem is still a stalk, but both together form an 
inseparable, and, moreover, altogether purely accidental organ 
for the plant, which has secreted the annual individual upon its 
surface, in order to bring into connexion, by means of a single 
organized membrane, the whole sum of the newly formed 
young individuals. The tree corresponds precisely to the 
polypidom, and it appears to me to be as unsound to set out 
from it as the type in plants, as it would be for the zoologist 
to take a Gorgonia as the ideal of animal individuality. This 
analogy, however, is in no way weakened by the circumstance, 
that we meet with this woody stem most frequently in precisely 
the highest developed plants ; but, on the contrary, it is natural 
that, if the animal kingdom in a certain measure receive the 
vegetative part of its character from the vegetable kingdom, 
this should connect itself by the lowest stage of animals to the 
highest plants, whilst even this vegetative half of the vital phe- 
nomena in the higher animals is in like manner purified and 
ennobled by its individuality constantly gaining in independence. 

With this explanation of the woody stem (the root included), 
it will henceforward appear by no means remarkable that this 
organ (as if it were a mere organized soil) can generate upon 
every part of its surface young vegetable individuals; that is, 
buds, so soon as it is in a condition to convey nutritive material 
to those buds from any part, whether it correspond apparently 
to the former root or to the stem; while this refined idea of 
the plant conducts to the law, that in the regular course of 
vegetation, neither root nor internode, but only the axilla of 
the leaf, is capable of generating a bud, i. e., a new axis with 
lateral organs. 


PHYTOGENESIS. 261 


But the following remarks, which in nature (who never, like 
a bad artist without a plan, fluctuates between the most oppo- 
site methods) would be, in the usual mode of treating it, an inex- 
plicable contradiction and an absolute miracle, will serve for the 
decisive establishment of this view. 

So soon as the secretion of this organized mass, the wood, 
takes place, for instance, we suddenly miss the influence of the 
law of formation, which, until then, had without exception 
directed the growth of the entire plant in all its parts. Here, 
so far as we are at present acquainted with the subject, there is 
no formation of cells within cells, here no expansion on all sides 
of the originally minute vesicle occurs, there is here no cyto- 
blast upon which the young might be developed ; but beneath 
the outermost layer of cells, which are comprised in the term 
bark, an organisable fluid is poured out, as it were, into a single, 
large, intercellular space, which fluid, as it seems, consolidates 
quite suddenly throughout its entire extent into a new, alto- 
gether peculiarly-formed tissue of cells, which are deposited 
one upon another, the so-called prosenchyma. Here, more- 
over, there is decidedly no formation of vascular bundles from 
cells of lower dignity, for all of them originate simultaneously 
and of their full size; and what has been called (spiral) vessels 
of the wood, is something which differs immensely from the 
spiral vessels of herbaceous plants, both in respect of their 
origin, and probably of their physiological signification also.' 
In like manner, no result has been obtained from the con- 
troversies which have been sometimes carried on with great 
warmth respecting the function of spiral vessels, nor could 
any be gained, because each party meant the spiral vessels of 
herbaceous plants, or of the wood, ad libitum, completely 
losing sight of the possibility that the two might be very 
different things. If, for instance, we examine the cambium in 
the earliest period at which it begins to acquire organisation, 


' This position has undergone essential modifications, in consequence of subsequent 
researches which I have made with respect to the cambium, and which proved that 
a cambium, in the sense in which it had been previously used in physiology, namely, 
as denoting an amorphous formative fluid between the wood and bark, had no exist- 
ence at all; that the wood and the bark, on the contrary, form one uninterrupted 
continuity, and their margin is merely denoted by a layer of delicately-walled, gela- 
tinous cellular tissue. 


262 CONTRIBUTIONS TO 


we find that it consists throughout of gelatinous prosenchy- 
matous cells which perfectly resemble one another. Shortly 
afterwards, some separate longitudinal rows of these cells ap- 
pear to have increased somewhat in breadth, which is the only 
circumstance that distinguishes them from the adjacent mass. 
As development advances, we observe that some dark spots 
appear upon the walls of some of these expanded cells, which 
we soon recognise to be small, flat air-bubbles, that have been 
formed between their walls and those of the neighbouring cell. 
Gradually all the expanded cells which are so disposed one 
upon the other are changed in this way: the air-bubble gra- 
dually appears more sharply defined, assuming the circular or 
oval figure; and there appears in its centre a smaller circle 
which constantly becomes more distinct, and which originates 
in the following manner: when the deposition of new masses 
takes place upon the inner wall of the cell, the parts corre- 
sponding to the outer air-bubble remain free from the deposit, 
thus forming a small canal which traverses the newly-deposited 
mass. We now recognise the fully-developed porous vessel, 
the partition walls between each two superincumbent cells ap- 
pearing at the same time to be more or less absorbed. This 
history of the formation of the porous vessels, which may 
readily be observed in limes and willows, greatly contradicts 
the general notion that the porous canals serve to facilitate the 
communication of the sap. As the air-bubble is first formed 
on the outside of the wall, it renders the passage of the sap at 
that spot impossible, and for this reason the origin of the 
porous canal might be most readily and naturally explained as 
a local atrophy of the cell-wall. At the same time the above 
shows that the distinction between fir-wood and that of trees 
which bear leaves, in respect to anatomical structure, cannot 
be of such vast physiological importance; since, with similar ele- 
ments and development, the distinction is really based on the 
larger or smaller number of cells that are converted into porous 
vessels. 

There are still, however, a great many gaps to fill up. In 
particular the origin of the medullary rays, and their relation to 
the wood; the formation of the new bark; and, lastly, the origin 
of the buds in the body of the wood, are so many questions 
for extended researches, to the execution of which, however, we 


PHYTOGENESIS. 263 


may look forward at no distant time, when we consider the 
ardent and gratifying zeal which has been awakened and 
cherished, especially amongst our contemporaries, in favour of 
the sound and scientific study of the anatomy and physiology 
of plants. 


I have attempted in this Memoir, so far as lay in my power, 
to solve many interesting questions in Vegetable Physiology, or, 
by more accurate definitions of the subject, to advance nearer 
to a future solution. May these observations meet with a 
friendly reception at the hands of the vegetable physiclogists 
of Germany, and be speedily improved upon and extended. 


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Fig. 1. 


EXPLANATION OF THE PLATES. 


SCHLEIDEN’S TREATISE. 


PLATE I. 


Cellular tissue from the embryo-sac of Chamedorea 
Schiedeana in the act of formation. a. The inner- 
most mass, consisting of gum with intermingled 
mucous granules and cytoblasts. 6. Newly formed 
cells, still soluble in distilled water. c-e. Further 
development of the cells, which, with the exception 
of the cytoblasts, may still coalesce, under slight 
pressure, into an amorphous mass. 


. The formative substance from fig. |, a, more highly 


magnified, gum, mucous granules, nuclei of the 
cytoblasts, and cytoblasts. 


. A single and as yet free cytoblast, still more highly 


magnified. 


. A cytoblast with the cell forming upon it. 
. The same, more highly magnified. 


. The same. The cytoblast here exhibits two nuclei, 


and is delineated in 
isolated after the destruction of the cell by pressure. 


. The same cellular tissue in a higher degree of deve- 


lopment than that represented by fig. 1, e. The 
contiguous cell-walls have already united. By 
making a transverse section, it may be distinctly 
perceived that the cytoblast is enclosed in the cell- 
wall. 


. Cells from a delicate transverse section of the almost 


matured albumen. 


266 EXPLANATION OF THE PLATES. 


Fig. 10. Common partition-wall between two cells from fig. 9, 
under a higher magnifying power. “The stratiform 
depositions may be observed at J, and the porous 
canals produced by their local failure at a. I could 
distinctly enumerate from nine to twelve layers 
which had been deposited within fourteen days. 


11. A sporule from Rhizina levigata Fries, with the 
cytoblast. 


12, 18, 14. Different cytoblasts from the embryo-sac of 
Pimelea drupacea before the appearance of cells. 


15. A young cell with its cytoblast, from the same. The 
latter in this instance presents the unusual number 
of three nucleoli. 


16. A portion of the embryonal end of the pollen-tube 
projecting from the ovulum in Orchis Morio, within 
which, towards the upper part, cells have been 
already developed. At the lower part, the original 
pollen-tube may still be distinguished. The almost 
globular cytoblasts are, in this instance, distinctly 
enclosed in the cell-wall. 


17. Embryonal end of the pollen-tube from Linum pal- 
lescens, together with an appended lobule of the 
embryo-sac (a). The process of the formation of 
cells is commencing. Above, a young cell with its 
cytoblast is already perceptible, beneath this several 
cell-nuclei are seen floating free. 


18, 19, 20. Commencing germination in the sporules of 
Marchantia polymorpha. Compare the text, p. 248. 


21. Portions of the pollen-tube which have become cel- 
lular, from Orchis latifolia, in the highest stage of 
development ; the investment of the pollen-tube is 
no longer perceptible. The cytoblast is enclosed in 
the cell-wall, just as in fig. 16. 


22 and 23. Two isolated cells from the terminal shoot 
(punctum vegetationis, Wolff) of Gasteria racemosa; 
22 exhibits two free cytoblasts; 23, two newly- 
formed cells within the original cell. 


J, Schieiden, adnat delt 


EXPLANATION OF THE PLATES. 267 


Fig. 24, A very young leaf of Crassula portulaca, the five 
cells which solely compose it being still surrounded 
by a parent-cell. 


25. Three cells from an articulated hair of potato, with 
a retiform current of mucus upon their walls. In 
the central cell the direction of the currents is par- 
tially indicated by arrows. 


In all the instances in which I have observed the movements in the cells of phz- 
nogamous plants, I have constantly found the moving matter to consist of a yellowish 
mucous fluid, perfectly insoluble in distilled water, and mixed with minute black 
granules, but differing entirely from the other aqueous sap of the cells; and even 
when the currents were so small as to appear merely as excessively minute delicate 
lines of black points, I sueceeded with higher magnifying powers in distinguishing 
the yellowish mucous fluid, especially when aided by the favorable circumstance 
(which not unfrequently occurs) of the current becoming arrested by some impedi- 
ment, which causes a somewhat larger quantity of the moving material to accumu- 
late, and is generally followed either by a change in the direction, or a division of 
the current. 


PLATE II. 


Fig. J. Cells from the epidermis of the pericarp of Ocymum _ 
basilicum, moistened with water, so that the mucous 
globule has expanded, and torn the outer cell-wall 
(a) from the side walls (4). 


2. Cells from the pericarp of the epidermis of Ziziphora 
dasyantha. 

3. Cells from the pericarp of the epidermis of Salvia ver- 
ticillata. 

4. Cells from the pericarp of the epidermis of Salvia 
Horminum. 

5. Cells from the pericarp of the epidermis of Salvia 
Spielmanni. 

2, 3, 4 and 5, a, exhibit the remains of the side-walls of 
the ruptured cells. 

6. A portion of the epidermis (a) and of the integument 
(2) of the ovule of Collomia coccinea. 'The epidermis- 
cells contain merely granules of starch. 


268 EXPLANATION OF THE PLATES. 


Fig. 7. The epidermis-cells of the half-ripe seed of the same 
plant, for the most part containing gum; at a, some 
still undecomposed starch. 


8. The same cells from the same seed nearly ripe. Beau- 
tiful spiral fibres have been formed from the con- 
tents, which are entirely consumed. 


9. Cells of the epidermis of the seed of Leptosiphon andro- 
saceum, moistened with water, so that the cone of 
jelly has come forth. a. The remains of the cell- 
walls. 


10. Cells from the epidermis of the seed of Hydrocharis 
Morsus rane. In the lower part of the cells, where 
they are connected together, the spiral coils take a 
direction different from that in the upper and free 
part. 


For the figures in Plate II consult the text, pp. 243-6. 


THE END. 


C. AND J. ADLARD, PRINTERS, 
BARTHOLOMEW CLOSE. 


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